The influence of ring-puckering on the light-induced ring-opening dynamics of heterocyclic compounds was studied on the sample 5-membered ring molecules gamma-valerolactone and 5H-furan-2-one using time-resolved photoelectron spectroscopy and ab initio molecular dynamics simulations. In gamma-valerolactone, ring-puckering is not a viable relaxation channel and the only available reaction pathway is ring-opening, which occurs within one vibrational period along the C-O bond. In 5H-furan-2-one, the C = C double bond in the ring allows for ring-puckering which slows down the ring-opening process by about 150 fs while only marginally reducing its quantum yield. This demonstrates that ring-puckering is an ultrafast process, which is directly accessible upon excitation and which spreads the excited state wave packet quickly enough to influence even the outcome of an otherwise expectedly direct ring-opening reaction.