We present an experimental study of intermodulation distortion (IMD) in high-speed, long-wavelength GaInAsP lasers modulated up to 8 GHz. It is found that three-tone distortion products are the dominant contribution to IMD interference when the number of subcarriers is large. These distortion data are used with a simple theory to predict accurately the measured signal/noise ratio for each channel of a multichannel subcarrier system.