In this study, the thermal gelation points of a vinylidene chloride/acrylonitrile random copolymer have been determined using a spectrophotometric technique. In the first part, the thermal gelation temperature and the thermal gelation weight loss have been determined as 219-degrees-C and 4.6%, respectively. In the second part, isothermal gelation times at 150, 160, 170 and 180-degrees-C have been determined as 256, 105, 44 and 27 min, respectively. At these temperatures the isothermal gelation weight losses have been found to be 0.37, 0.68, 0.92 and 1.26%, respectively. In conclusion, it has been shown that the crosslinking reaction in vinylidene chloride/acrylonitrile copolymer begins at lower temperatures and weight losses in isothermal heating than in dynamic heating.