The in situ UV photolysis of a precursor molecule trapped in a parahydrogen (pH(2)) matrix is a simple method used to generate isolated radical photofragments that are well suited for infrared spectroscopic studies. However, for molecules that can dissociate via multiple pathways, little is known about how the pH(2) matrix influences the branching among these open pathways. We report FTIR spectroscopic studies of the 193 nm photodecomposition of methylamine (MA, CH3NH2) isolated in pH(2) quantum matrixes at 1.8 K. We observe single exponential decay of the MA precursor upon irradiation and the quantum yield for MA photodissociation is measured to be F = 0.26(2) consistent with a weak pH(2) cage effect. By comparing to gas-phase results, we show the in situ photolysis results in greater production of molecular products (CH2-NH + H-2) compared to radical products (CH3NH + H) consistent with the idea of partial caging of the H atom photofragments. The information gained in this work can be used to guide future photolysis studies in pH2 matrixes.