The influence of sour substances (citric, acetic, lactic and tartaric acids) on the oscillation pattern of liquid membrane (nitromethane) oscillators containing cationic (benzyldimethyltetradecylamonium chloride) or anionic (sodium oleate) surfactant was investigated. The citric acid concentration effect on oscillation curves has been also examined. It was shown that oscillation patterns were different in the presence of appropriate sour substances in cationic or anionic oscillators. The time delay method was used for reconstruction of attractors in two-dimensional phase-space based on oscillation curves of these systems. The analysis of appropriate attractors might be used efficiently for discrimination of sour substances. They might be used as a "fingerprint". However, no direct correlation between the surface of appropriate attractors and citric acid concentration has been found.