INA-Naftaplin has been utilizing the LO-CAT® process in Croatia for protection of ambient air against H2S pollution at its Gas Treatment Plant Molve III already for seven years. This separation unit, incorporated into the gas treatment plant has been erected exclusively for the protection of ambient air against harmful effect of H2S. The unit is treating H2S+CO2, which is being removed from natural gas by an upstream amine process, under conditions of low H2S content and low pressure of gas. Catalytic oxidation is being used to convert H2S to elementary sulfur, and the emission concentration was decreased from 580 ppm to less than 30 ppm. The practice revealed a series of delicate situations: plugging of internals, solution filtering and achievement of required 60 wt% sulfur concentration, disposal of produced sulfur sludge and others. The Institute for Medical Research Zagreb has performed the ambient air quality monitoring, within the scope of its contract for annual base monitoring. H2S imission values were under 5 μg/m3. The continuous control of working area is achieved by twenty-four H2S sensors, and results obtained to this date have always been below allowable limits. The practical experience has revealed that the LO-CAT® desulfurization unit has fulfiled its purpose and existing environmental criteria till 1997, along with significant cost, primarily for power and chemicals. The actual Croatian Directive on TLV of pollutants in the waste gas requires H2S concentration of 3.5 ppmv max. and RSH concentration of 20 mg/m3 max. Now the GTP Molve and Ethane Recovery Plant have emission of H2S and mercaptane above the permissible limit value, which have to be solved until the year 2004. To meet the strict legal requirements, certain upgrading is to be undertaken. This capital investment requires a large financial expenditure of up to 5 million USD, and up to 1 million USD/year for operating cost for both plants. For process improvement a continuous monitoring system has to be solved also. In that respect, measuring of H2S+RSH emission is in accordance to ASTM D-4084-82 and ASTM D-23 85-81 methods. Periodical control by gas tube detector system could contribute to the most reliable and efficient monitoring system of the two most important natural gas plants in Croatia.