An excitation-pattern model [Moore and Glasberg, Hear. Res. 28, 209--225 (1987)] was used to predict the masking additivity of two simultaneous sinusoidal maskers. The prediction relies on the assumption that masked threshold is reached when the addition of the signal to the masker produces a critical difference in the masker excitation pattern (EP) at any center frequency (the critical EP difference is termed criterion EPD). For equally effective maskers, masking additivity was assumed be equal to the difference between two signal levels: The level that produces the criterion EPD with the combination of maskers minus the level that produces the criterion EPD with the individual maskers. Excitation-pattern predictions of masking additivity were obtained for signal frequencies of 500, 1000, and 5000 Hz, each masked by two sinusoidal maskers symmetrically placed about the signal frequencies; maskers frequencies within and outside the signal ERB were considered. The masking additivity predicted by the EP model is compared to that predicted by the modified power law model [Humes et al., J. Acoust. Soc. Am. 83, 188--202 (1988)], and to masking-additivity data [Lutfi, J. Acoust. Soc. Am. 73, 262--267 (1983)].