Inverse yield–density relationships were used to evaluate how competitive balances in mixed plant species associations may influence productivity, as measured by three indices : Relative Yield Total (RYT), Relative Land Output (RLO), and Total Land Output (TLO). Parameters of the inverse relationships, which express competitive influences and plant growth potential in the absence of competition, were incorporated into expressions used to predict RYT, RLO and TLO. Initial parameter values were derived from 25 experiments on binary species mixtures, and parameter values were systematically varied over a 100-fold range. Response to parameter manipulation was also investigated in five specific binary associations representing a broad range of relative mixture productivity. As indicated by RLO or RYT, and in accord with ecological concepts of niche differentiation, high productivity of mixtures relative to monocultures occurred if between-species competition was low. As indicated by TLO, the total combined productivity of the mixed species was enhanced by higher potential growth per plant in the absence of competition. Lower within- and between-species competition also increased TLO. There was a significant positive correlation between RYT and RLO. Relative and total measures of mixture productivity, however, showed different responses to parameter manipulation, and were not correlated.