Current greenhouse gas accounting standards allow companies to use renewable energy certificates (RECs) to report reductions in emissions from purchased electricity (scope 2) as progress towards meeting their science-based targets. However, previous analyses suggest that corporate REC purchases are unlikely to lead to additional renewable energy production. Here we show that the widespread use of RECs by companies with science-based targets has led to an inflated estimate of the effectiveness of mitigation efforts. When removing the emission reductions claimed through RECs, companies’ combined 2015–2019 scope 2 emission trajectories are no longer aligned with the 1.5 °C goal, and only barely with the well below 2 °C goal of the Paris Agreement. If this trend continues, 42% of committed scope 2 emission reductions will not result in real-world mitigation. Our findings suggest a need to revise accounting guidelines to require companies to report only real emission reductions as progress towards meeting their science-based targets.