1. INTRODUCTION The results from a third structure determination by powder diffractometry (SDPD) round-robin are summarized. From the 175 potential participants having downloaded the powder data, 9 have sent a total of 12 solutions (8 and 4 respectively for samples 1 and 2, a calcium tartrate tetrahydrated and a lanthanum tungstate). They used 7 different computer programs for structure solution (ESPOIR, EXPO, FOX, PSSP, SHELXS, SUPERFLIP, TOPAS), applying either the Patterson or direct methods, direct space methods, or the charge flipping approach. INTRODUCTION The results from a third structure determination by powder diffractometry (SDPD) round-robin are summarized. From the 175 potential participants having downloaded the powder data, 9 have sent a total of 12 solutions (8 and 4 respectively for samples 1 and 2, a calcium tartrate tetrahydrated and a lanthanum tungstate). They used 7 different computer programs for structure solution (ESPOIR, EXPO, FOX, PSSP, SHELXS, SUPERFLIP, TOPAS), applying either the Patterson or direct methods, direct space methods, or the charge flipping approach. CONCLUSION Solving a structure from powder data remains a hard job, an order of magnitude harder than solving a problem with similar complexity from single crystal data. Nevertheless, a few more steps in the direction of routine were accomplished since the two previous round-robins : that time, not only the computer program developers were successful, but also some more standard users. No result was obtained from crystal structure prediction experts. CONCLUSION Solving a structure from powder data remains a hard job, an order of magnitude harder than solving a problem with similar complexity from single crystal data. Nevertheless, a few more steps in the direction of routine were accomplished since the two previous round-robins : that time, not only the computer program developers were successful, but also some more standard users. No result was obtained from crystal structure prediction experts. REFERENCES Adil, K., Le Bail, A., Dujardin, G. & Maisonneuve V. (2007). Polyhedron 26, 2493- 2497. Chambrier M.H., Le Bail A. & Goutenoire F. (2008). http://www.crystallography.net/ Clegg, W. & Teat, S.J. (2000). Acta Cryst. C56, 1343-1345. Le Bail A. (2003). COD1000005 - http://www.crystallography.net/ Le Bail, A. & Cranswick, L.M.D. (2001). Commission on Powder Diffraction IUCr Newsletter 25, 7-9. Le Bail, A. & Cranswick, L.M.D. (2003). Commission on Powder Diffraction IUCr Newsletter 29, 31-34. Le Bail, A., Bazin, D., Daudon, M. & Maisonneuve, V. (2008). http://www.crystallography.net/ Zhu, J.H., Wu, H.X. & Le Bail, A. (1999). Solid State Science, 1, 55-62. REFERENCES Adil, K., Le Bail, A., Dujardin, G. & Maisonneuve V. (2007). Polyhedron 26, 2493- 2497. Chambrier M.H., Le Bail A. & Goutenoire F. (2008). http://www.crystallography.net/ Clegg, W. & Teat, S.J. (2000). Acta Cryst. C56, 1343-1345. Le Bail A. (2003). COD1000005 - http://www.crystallography.net/ Le Bail, A. & Cranswick, L.M.D. (2001). Commission on Powder Diffraction IUCr Newsletter 25, 7-9. Le Bail, A. & Cranswick, L.M.D. (2003). Commission on Powder Diffraction IUCr Newsletter 29, 31-34. Le Bail, A., Bazin, D., Daudon, M. & Maisonneuve, V. (2008). http://www.crystallography.net/ Zhu, J.H., Wu, H.X. & Le Bail, A. (1999). Solid State Science, 1, 55-62. The third Structure Determination by Powder Diffractometry Round Robin (SDPDRR-3) L.M.D. Cranswick and A. Le Bail Canadian Neutron Beam Centre, National Research Council Canada, Building 459, Chalk River Laboratories, Chalk River ON, Canada, K0J 1J0. Email : lachlan.cranswick@nrc.gc.ca Université du Maine, Laboratoire des Oxydes et Fluorures, CNRS UMR 6010, Av. O. Messiaen, 72085 Le Mans, France. Email : Armel.Le_Bail@univ-lemans.fr The third Structure Determination by Powder Diffractometry Round Robin (SDPDRR-3) L.M.D. Cranswick and A. Le Bail Canadian Neutron Beam Centre, National Research Council Canada, Building 459, Chalk River Laboratories, Chalk River ON, Canada, K0J 1J0. Email : lachlan.cranswick@nrc.gc.ca Université du Maine, Laboratoire des Oxydes et Fluorures, CNRS UMR 6010, Av. O. Messiaen, 72085 Le Mans, France. Email : Armel.Le_Bail@univ-lemans.fr Sample 1 : CaC 4 H 4 O 6.4H 2 O Sample 2 : La 18 W 10 O 57 Conventional X-rays SynchrotronNeutron