The global threat of plant invasions to native ecosystems mandates an understanding of the mechanisms that determine invasion success. While some exotic species establish, spread and impact native ecosystems, others establish with little or no measurable impact. Competitive ability has been highlighted as a key mechanism influencing invasion success and impact, but there is growing evidence that interactions between plants and soil biota may also be important. In particular, escape from natural enemies during the early stages of establishment may give exotic species an advantage over native species subject to their own natural enemies in the soil. Here, we evaluated whether the invasion success and impact of exotic grass species could be explained by competitive superiority over resident native species, advantages gained from the positive effects of native soil communities, or both. We assessed the competitive abilities of six grass species that vary in their invasion success relative to three widespread native grasses, quantified the effects of native soil microbial communities on the performance of both native and exotic grass species, and determined whether there was an interaction between competitive and soil effects. Overall, we found that exotic species were stronger competitors than natives and that native soil effects were weak and did not predict invasiveness. Differences in species relative competitive abilities also did not correlate with invasiveness but demonstrate how some exotic species could outcompete natives and suppress their growth during the invasion process.