Optical submarine systems are now being planned, and because of the need for high levels of confidence in such systems, all aspects that affect performance need detailed investigation. Of particular importance are penalties that may arise from spectral effects. A theoretical and practical examination has been made of the relationship between source spectrum and system performance in the presence of chromatic dispersion. Equipment is described which provides an automatic record of the power budget penalty resulting from spectral variations for all temperatures in the operating range. Significant differences are found, even between lasers of the same type. The nature of the corresponding spectral differences is studied, and evidence is presented to show that both random and regular variations exert appreciable effects.