After graduating, Gouterman was appointed to the faculty at Harvard University where he worked as a postdoctoral researcher with William Moffitt. Shortly after Gouterman arrived, Moffitt died of a heart attack during a squash game. Gouterman was quickly promoted to assistant professor, and spent his time using quantum chemical calculations to understand the photophysical properties of porphyrins. He primarily made use of the Hückel molecular orbital method to interrogate their optical spectra. Gouterman's molecular models, which included symmetry arguments and configuration interactions, were able to predict the intensity differences between the absorption bands of porphyrins. The so-called four-orbit model incorporates two, almost degenerate highest occupied molecular orbitals and two degenerate lowest unoccupied molecular orbitals. The Soret and Q-bands that are visible in porphyrin spectra are the result of transitions from between these four orbitals. 
Gouterman moved to the University of Washington in 1966, where he worked until his retirement. In Seattle, Gouterman continued to study the optical properties of porphyrins. He described how the chemical structures of porphyrins determine whether the spectral shape was normal, hyper- and hypso-. For example, the UV-Visible absorption spectra of hyper porphyrins contain red-shifted peaks and extra bands due to ligand-to-metal charge transfer (LMCT) transitions. Amongst the complicated structures analysed by Gouterman were cytochrome P450–carbon monoxide complexes, whose electronic spectra included a split Soret band due to LMCT transitions.