Using the beam propagation method of analysis, the perlormance of a parallel configuration of integrated optical Mach-Zehnder interferometers is examined. These configurations require the interferometers to be spaced closely together to limit the length of the substrate. As a result, the leakage or substrate radiation modes that exist outside each interferometer extend to the point where interference with adjacent substrate radiation modes (and, hence, guided optical power in close proximity) occurs. The radiation mode coupling is shown to have a significant effect on the performance of the interferometer array. The induced change in the mode propagation constants (Δβ) and the corresponding insertion loss due to the radiation mode interference are evaluated numerically as a function of the separation distance between the interferometers. Distortion in the modulation characteristics due to the radiation mode coupling is also quantified and compared with the theoretically expected values.