We show here that the inefficiency of S-1(A(2)) --> S-0(A(1)) and S-2(A(1)) --> S-1(A(2)) internal conversion in gaseous thiophosgene (Cl2CS) has its most likely origin in the absence of an a(2) vibration that can vibronically couple S-1 with S-0 and S-2. The finding confirms the critical role the promoting mode plays in electronic radiationless transitions.