Photochromic polycyanurates containing mercury dithizonate moiety have been prepared by interfacial polycondensation technique. All the polycyanurates have been characterized by their IR spectra, viscosity, molecular weight by VPO and thermogravimetry. The film of PMMA blended with photochromic polycyanurate shows a very good photochromic effect on exposure to sunlight and regains its original colour on removal of sunlight. The changes of return to the original colour have been measured using Beckman DK-2A Ratio Recording spectrophotometer.