Coherent phase control of the photodissociation of CH3I has been achieved by quantum mechanical interference between competing paths. The control was accomplished by exciting the parent molecules with three UV photons of frequency omega(1) and one VUV photon of frequency omega(3)=3 omega(1). Varying the phase difference between the two laser beams resulted in a modulation of the I+ and CH3+ signals, without affecting the parent ion signal. We propose a mechanism in which control occurs over the photodissociation step to produce CH3+I*, followed by ionization of the neutral fragments by additional UV photons.