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The isotopic interaction between ambient vapor and raindrops has been investigated for the first time at Port Blair, Andaman Islands, an environment having the minimal contribution of the continental moisture. Rainwater and ambient vapor samples were collected on a daily timescale during the Indian summer monsoon season of 2015. Oxygen (Hydrogen) heavy isotopic ratios of rainwater and ambient vapor are positively correlated and a difference in the seasonal average values was found to be 9.5%0 (65.8%o) at similar to 28 degrees C. On a daily scale, the oxygen isotope ratios of ambient vapor and rainwater are significantly correlated over a wide range of rainfall amount, but correlation weakens for rainfall exceeding similar to 36 mm/day. The isotopic variability of rainfall appears to be modulated by the interaction with the ambient vapor, which in turn is determined by the source moisture. In a case study, it is estimated that the vapor-rainwater isotopic exchange could lead to O-18 enrichment (depletion) in raindrops (vapor) about 0.41%0 (approx. 30% of the total change), which is far above ( > 4 sigma) of the experimental uncertainty.