We have measured the intersystem crossing (ISC) rates of optically excited triplet pyrazine-d(4) in supersonic expansion. ISC rates ranging from 3.3X10(2) to 1.3X10(3) were found in the 1153 cm(-1) range of excess vibrational energy above the T-1 origin. These rates were substantially lower than the ISC rate of pyrazine. Our measurements were accompanied by model calculations of the ISC rates of pyrazine and pyrazine-d(4). According to this model, certain vibrational modes, which undergo large frequency reduction in the excited state, regulate the strong vibrational energy dependence of the T-1-->S-0 ISC of pyrazine. The large frequency changes result from the interaction of the near lying (3)n pi(*) and (3) pi pi(*) states (the "proximity effect"). These calculations account quantitatively for the excess vibrational energy dependence of the ISC rates in pyrazine, as well as for the isotopic substitutional effect.