The heat, ΔH=–28·7 ± 1·6 kcal./mole, of the reaction trans-(Et3P)2PtPh2(cryst.)+ HCl(g)→trans-(Et3P)2PtPhCl(cryst.)+ C6H6(g) has been obtained by use of a Calvet microcalorimeter. The difference, ΔH= 30·0 ± 3·0 kcal./mole, between the bond energies E(Pt–Cl)–E(Pt–C) is calculated. Making a reasonable estimate of 91 kcal./mole for E(Pt–Cl), we derive a bond energy of about 60 kcal./mole for E(Pt–C).