Photodissociation dynamics of methyl iodide (CH3I) adsorbed on both amorphous solid water (ASW) and porous amorphous solid water (PASW) has been investigated. The ejected ground-state I(P-2(3/2)) and excited-state I(P-2(1/2)) photofragments produced by 260- and 290-nm photons were detected using laser resonance-enhanced multiphoton ionization. In contrast to gas-phase photodissociation, (i) the I(P-2(3/2)) photofragment is favored compared to I(P-2(1/2)) at both wavelengths, (ii) I(P-2(3/2)) and I(P-2(1/2)) have velocity distributions that depend upon ice morphology, and (iii) I-2 is produced on ASW. The total iodine [I(P-2(3/2))+ I(P-2(1/2))+ I-2] yield varies with substrate morphology, with greater yield from ASW than PASW using both 260- and 290-nm photons. Temperature-programmed desorption studies demonstrate that ice porosity enhances the trapping of adsorbed CH3I, while pore-free ice likely allows monomer adsorption and the formation of two-dimensional CH3I clusters. Reactions or collisions involving these clusters, I atomic fragments, or I-containing molecular fragments at the vacuum-surface interface can result in I-2 formation. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790585]