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Experiments are described to show some of the potential of the synchrotron radiation Laue method for the study of structural change within single crystals. In the metastable tetragonal crystals of P4N4Cl8 the eight-membered P4N4 ring is in a boat conformation, with symmetry 
. On heating to ca 340 K the crystals transform, slowly to a second tetragonal form in which the ring conformation is a chair, its symmetry 
. Both structures are known [Hazekamp, Migchelsen & Vos (1962). Acta Cryst. 15, 539-543; Wagner & Vos (1968). Acta Cryst. B24, 707-713]. In the transformation the molecular packing, unit-cell dimensions and crystal quality remain almost unchanged. To study this transformation, series of Laue diffraction patterns were recorded at 2-3 min intervals over a period of 30-40 min, while the temperature was raised to 373 K. For two series, reflection intensities were measured and they allowed determination and refinement of the fraction of boat and chair molecules present in a mixed boat/chair model of the structure. No significant change in the crystal occurs below ca 340 K; at or above 340 K, 40-50% of the molecules are converted from boat to chair conformations within 5 min, but the remainder of the conversion is much slower, even when the temperature is raised towards 370 K.
. On heating to ca 340 K the crystals transform, slowly to a second tetragonal form in which the ring conformation is a chair, its symmetry 
. Both structures are known [Hazekamp, Migchelsen & Vos (1962). Acta Cryst. 15, 539-543; Wagner & Vos (1968). Acta Cryst. B24, 707-713]. In the transformation the molecular packing, unit-cell dimensions and crystal quality remain almost unchanged. To study this transformation, series of Laue diffraction patterns were recorded at 2-3 min intervals over a period of 30-40 min, while the temperature was raised to 373 K. For two series, reflection intensities were measured and they allowed determination and refinement of the fraction of boat and chair molecules present in a mixed boat/chair model of the structure. No significant change in the crystal occurs below ca 340 K; at or above 340 K, 40-50% of the molecules are converted from boat to chair conformations within 5 min, but the remainder of the conversion is much slower, even when the temperature is raised towards 370 K.
