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The least-squares refinement of H parameters on highorder X-ray data shows that the data do contain information about the H atoms, but in this study the amount of data seems to be less than that needed for satisfactory refinement. The variation in the N-H bond lengths with the various minimum sin 0/2 cut-off values follows the same trend as that seen in diformohydrazide" the N-H distances decrease with increasing (sin 0/,~,)min up to about 0.50 A -l, where the shortest bond lengths are found; a minimum cut-offof 0.75/~-1 is necessary in order to obtain a reasonable mean N-H bond length.The excellent agreement between the difference electron densities obtained for carbonohydrazide and diformohydrazide, where the differences follow the expected electronic variations, further support the usefulness of the X -Xao method for obtaining information about electron density deformations. References COMPTON, A. H. & ALLISON, S. K. (1935). X-rays in