Combined theoretical and experimental studies of rotationally resolved photoelectron spectra for single-photon ionization of the outermost valence orbitals of H2O, H2S, H2CO, and CH3 are reported. Agreement between calculated and measured spectra is very encouraging. Both show that photoionization dynamics is very molecular in origin for H2O, H2S, and H2CO but quite atomiclike for CH3. Parity selection rules and the angular momentum composition of the photoelectron are used to illustrate the dynamical aspects of photoionization of polyatomics as molecular symmetry changes in a group of structurally related systems.