Kinetic information extracted from biochemical methane potential (BMP) tests is often reported but its value is unclear. Inter-laboratory reproducibility provides a useful indication of its value. Here we extracted estimates of the first-order rate constant 𝑘 from 1259 methane production curves collected in a large inter-laboratory study on BMP in order to quantify reproducibility. Reproducibility in 𝑘 was poor; relative standard deviation was 50–140%. Substrate comparisons ( 𝑘 for one substrate compared to another) also had low reproducibility, regardless of low 𝑝 values from inferential statistical tests. The use of a shared inoculum did not improve reproducibility in 𝑘 . We conclude that 𝑘 estimates from BMP tests only partially reflect intrinsic substrate properties. Therefore, interpretation and application of batch kinetic results should be done cautiously.