A model of the effect of interspecies competition for light on the daily dry-matter production of competing species is developed. It describes the absorption of light by the component species, accounting for their different optical properties. Absorption equations are formulated from the principle of conservation of light energy within a unit-leaf-area-index of canopy, and the integral of absorbed light energy is then used to calculate the rate of dry-matter production of each component species. This model proved useful for the analysis of data from experiments in which New Zealand white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were grown in mixtures. Application of the model enabled the proportion of incident light energy absorbed by each component to be estimated and unambiguous differentiation between the amount of light energy absorbed and the efficiency of use of the absorbed light energy.