Comparison of crystal structure parameters obtained for ThCl4 from quantitative X-ray powder data with those available from single-crystal studies has shown that it is possible to obtain reliable parameters from powder studies on simple compounds. Structure refinement based on powder data for ThCl4(R= 8·9%), PaCl4(R= 7·6%), UCl4(R= 9·5%), NpCl4(R= 9·1%), and ThBr4, (R= 11·3%) has yielded crystal structure parameters, and bond lengths for these isostructural compounds. Accurate unit cell dimensions are also reported.