Crystals of KNiPO4 have space group Pna21 at room temp. (a 8.6358(4), b 9.2594(5) and c 4.9096(3) .ANG.). This compd. has two structural phase transitions, at 485 Deg and 580 Deg. Ferroelec. 180 Deg domains are revealed by chem. etching with a 0.1M HCl soln. on {001} cuts. A single domain was cut from an etched crystal and an x-ray study of the abs. structure showed that the (001) facets are preferentially dissolved by the etchant. Another crystal having the two ferroelec. domain orientations in a ratio 31.3:68.7 was also cut from the same etched sample and studied by x-ray diffraction. The abs. structure parameter xabs, available in the Xtal 3.2 program was refined together with the other variables. This parameter gives the ratio of the crystal which has its abs. structure reversed with respect to the model. A final value of xabs = 33.2(1.8)% for the twinned crystal was found. [on SciFinder (R)]