Time-resolved infrared spectroscopy was used to study the photoisomerization of N-Methylthioacetamide (NMTAA) in D2O in both the cis -> trans and the trans -> cis direction upon selective excitation of the n-pi* (S-1) and pi-pi* (S-2) electronic transitions. While isomerization and the return to the ground state takes place on two distinct time scales (<= 8 ps, similar to 250 ps) upon pi-pi* excitation of both cis- and trans-NMTAA in D2O, ground state recovery is only observed on the slower time scale upon n-pi* excitation. The quantum efficiency for trans -> cis isomerization is 30-40%, independent of the electronic state excited, while the cis -> trans isomerization proceeds with a 60-70% quantum efficiency. These results support a mechanism by which isomerization takes place via one common intermediate state independent of electronic excitation energy and initial conformation.