We present pressure-dependent subthreshold photoionization spectra of CH3I doped into varying number densities of the perturber gases Ar, N-2 and CO2. The intensity of the observed subthreshold structure is discussed in terms of two different interactions, namely electron attachment and associative ionization. Effective rate constants for these two processes are analyzed, and the variation in these constants is discussed in terms of the properties of the dopant excited state.