 This study examined the reproducibility of off-axis integrated cavity output spectroscopy, OAICOS, measurements of deuterium and oxygen isotopic compositions of water samples when compared to those obtained via mass spectrometry. Four OAICOS instruments were used to measure the samples, and their performance was evaluated by comparing the results to those obtained by mass spectrometry. The results showed that the OAICOS instruments had good reproducibility, with a mean difference of less than 2% between the two methods. Additionally, the results showed that the OAICOS instruments could be used to accurately replicate the mass spectrometry results if the same sample was measured multiple times. This suggests that OAICOS can provide reliable and accurate measurements of deuterium and oxygen isotope composition of water samples. This article was authored by Mbonaza, BMC Fisher, Mborga, and others.