[en] A preliminary study of the isotope hydrology of the Kedah and Perlis area was undertaken under the RCA programme. This project is an attempt at elucidating the mechanism of recharge, origin, area of recharge and dating of groundwater system in the area. The results show that all groundwater samples in the area vary within a narrow range for ¹⁸0 (-7.58 to -5.06%) while ²H ranges from -50.3 to -35.1%. The mean isotopic composition of precipitation collected at Alor Star meteorological station fall within the range of variation of the Kedah/Perlis groundwaters. In the southern part of the study site, the isotopic results indicate the occurence of two types of water: firstly, the recharge is from the highlands where more negative ¹⁸0 values and low tritium were observed and the second type is of local recharge where high tritium and less negative ¹⁸0 values were observed. On the other hand, in the northern part of the basin the interpretation of stable isotopic results is quite difficult to be made at this stage. There was no correlation at all between tritium and ¹⁸0 whatsoever. Results of the tritium assay show that some of the groundwater samples are pre-nuclear in age as indicated by low tritium content. Consequently, a Carbon-14 investigation was carried out from a few selected sites and their ages were found to be in the range of about 3000 to 5000 years. (author)