The performance of 20 European laboratories involved in long-term non-methane hydrocarbon (NMHC) measurements was assessed with respect to ACTRIS and GAW data quality objectives. The participants were asked to measure both a 30-component NMHC mixture in nitrogen and whole air. The NMHCs were analysed either by GC-FID or GC-MS. Most systems performed well for the NMHC in nitrogen, whereas in air more scatter was observed. Reasons for this are explained in the paper.