The effect of molecular size on the site-specific fragmentation was investigated by exciting the N and O K shells of methyl cyanoformate (CH 3 OCOCN) and methyl cyanoacetate (CH 3 OCOCH 2 CN). The fragmentation patterns were essentially identical in the N and O core excitations of CH 3 OCOCN. Site and state dependent fragmentation was clearly observed in the core hole CH 3 OCOCH 2 CN: The fragment ions mainly observed at the O K edge excitation were CH 3 + , HCO + , CH 2 CN + , CH 3 OCO + , and COCH 2 CN + . At the N(1s) excitation the large CH 3 OCO + and COCH 2 CN + fragments were extremely depressed, and alternatively the small CH 3 + , HCO + , and CH 2 CN + ions were dominant. Especially at the π* C N <-N(1s) resonance transition the N + and CO + fragment ions were exclusively produced with yields of 40 and 15%, respectively. The total photoabsorption cross sections were measured in the N and O K regions and the observed peaks were tentatively assigned. Reflectron type time-of-flight mass spectrometry was employed to attain the mass separation M/ΔM=<100 at M/e~100 for the core excited molecules ejecting fragments with considerable kinetic energies.