Mixed cultivation of two or more crop species on the same piece of land at a given time may enhance crop yield stability. We assessed yield stability by first calculating the expected coefficients of variance (CV%) for cereal-legume mixtures grown across contrasting environments based on the corresponding yield variation of the mixture components grown separately in pure cultures; and then dividing the expected CV% by the observed CV% for each mixture. The ratio between the expected and observed CV% is the stability index, with values >1 indicating greater stability of the mixed crops and values <1 indicating greater stability of the crops grown separately. Stability indices for eight out of 11 barley/pea and wheat/faba bean mixtures grown across Europe were between 0.7 and 1.8 (mean 1.0), whilst two barley/pea mixtures had indices >3.7. The results are relevant in the discussion of crop mixtures as means to increase yield stability.