Cover crop mixtures are popular among farmers because of the perception that mixtures will provide a greater quantity (and possibly quality) of ecosystem services. We know that cover crop biomass is usually greater in diverse mixtures compared to component monoculture plantings, but information about ecosystem services provided by cover crop mixtures is lacking. The objective of this study was to assess the relative contribution of 18 cover crop species to mixture productivity and ecosystem services including, soil microbial activity, soil nitrogen retention, nitrogen fixation, and weed suppression. Six individual cover crop species and all possible five-way mixture combinations of those species were planted in each of three possible rotational phases (spring, summer, and fall) on two organic vegetable farms in central and northern Illinois. Soil microbial activity was greater in cover crop plots compared to bare fallow, but did not vary by species or mixture composition. Cover crops reduced soil nitrate by up to 70% compared to bare fallow by maturity, but again, there was no difference among species or mixture composition. Legume root nodule biomass per plant was usually not different between legume monocultures and mixtures; however, nodule biomass tended to decrease with decreasing legume shoot biomass in mixture. Weed abundance varied widely among species and mixture, and weed abundance increased when yellow mustard (spring), sudangrass (summer), and rye (fall) were removed from the mixture community. We did not detect allelopathic weed suppressive benefits of any species or mixture one month after residue incorporation in soil. Lastly, shoot biomass, soil microbial activity, and nitrogen retention were usually not different between cover crops and resident weed communities. Results from this study suggest that farmers can choose the least expensive cover crop species or mixture when the primary objective is to retain soil nitrogen or stimulate microbial activity.