Concern exists about the suitability of laser spectroscopic instruments for the measurement of the O-18/O-16 and H-2/H-1 values of liquid samples other than pure water. It is possible to derive erroneous isotope values due to optical interference by certain organic compounds, including some commonly present in ecosystem-derived samples such as leaf or soil waters. Here we investigated the reliability of wavelength-scanned cavity ring-down spectroscopy (CRDS) O-18/O-16 and H-2/H-1 measurements from a range of ecosystem-derived waters, through comparison with isotope ratio mass spectrometry (IRMS). We tested the residual of the spectral fit S-r calculated by the CRDS instrument as a means to quantify the difference between the CRDS and IRMS delta-values. There was very good overall agreement between the CRDS and IRMS values for both isotopes, but differences of up to 2.3 parts per thousand (delta O-18 values) and 23 parts per thousand (delta H-2 values) were observed in leaf water extracts from Citrus limon and Alnus cordata. The S-r statistic successfully detected contaminated samples. Treatment of Citrus leaf water with activated charcoal reduced, but did not eliminate, delta H-2(CRDS)-delta H-2(IRMS) linearly for the tested range of 0-20 % charcoal. The effect of distillation temperature on the degree of contamination was large, particularly for delta H-2 values but variable, resulting in positive, negative or no correlation with distillation temperature. S-r and delta(CRDS) - delta(IRMS) were highly correlated, in particular for delta H-2 values, across the range of samples that we tested, indicating the potential to use this relationship to correct the delta-values of contaminated plant water extracts. We also examined the sensitivity of the CRDS system to changes in the temperature of its operating environment. We found that temperature changes >= 4 degrees C for delta O-18 values and >= 10 degrees C for delta H-2 values resulted in errors larger than the CRDS precision for the respective isotopes and advise the use of such instruments only in sufficiently temperature-stabilised environments. Copyright (C) 2011 John Wiley & Sons, Ltd.