The partially resolved vibrational fine structure on the C 1s photoelectron line of C2H2 clearly differs from that of other small hydrocarbon molecules such as CH4, C2H4, and C2H6 This is attributed to photoionization from the two 1 sigma(g) and 1 sigma(u) core orbitals of C2H2, which can be described as the symmetric and antisymmetric linear combinations of the C 1s orbitals. According to our analysis they are split by 105 +/- 10 meV, a result which is in good agreement with calculation. [S0031-9007(97)04424-4].