The coordination complex [Ni(en)3]2+(NO3-)2 (en = 1,2-diaminoethane) undergoes a sharp reversible displacive phase transition at -109 K, changing space group from P6322 above the transition temperature to P6522 below. The phase change is accompanied by a tripling of the c axis on cooling, resulting in an easy detection of the transition in images from area-detector diffractometers. The transition has been followed using a Nonius KappaCCD and a Bruker SMART APEX CCD. Data sets were collected over the temperature range 100-113 K and integrated using the low-temperature orientation matrix. Reflections with ≠ 3n show a smooth and rapid decrease in intensity to zero on warming from 106.5 to 111 K. The results are reproducible to within ±2 K in two laboratories and suggest that this compound may be useful as a liquid-nitrogen cryo-calibrant for diffraction instruments equipped with area detectors.