Abstract Two methods used to investigate a small contamination of negative pions in intense muon beams available at the CERN 600 MeV Synchro-cyclotron are described. A total cross-section method yields a measure of the total contamination and, thanks to its speed, permits a rapid optimization of the beam. A star-counting method relies on the large pulses given out by negative pions when captured in a scintillator at the end of their range. It is relatively slow, but gives detailed information on the range spectrum of the contamination, which is of special value in capture experiments. Results obtained with both methods are described and compared. They indicate total contaminations of the order of 1% in the beams investigated. In the stopping range of the muons, contaminations of a few parts in 10 4 were observed.