Subthreshold photoionization spectra of CH3I perturbed by CF4 and c-C4F8 for variable CH3I number density rho (CH3I) and perturber number density rho (p) are presented. The rho (CH3I) and rho (p) dependence of the subthreshold structure is discussed in terms of the excited-state processes of electron attachment and associative ionization. The effective rate constants for these processes are determined from the variation in the subthreshold photocurrent as a function of rho (CH3I) and rho (p). These constants are then analyzed with respect to the properties of the excited CH3I Rydberg state. The variation in the effective rate constants is discussed in terms of ground-state perturber properties.