Chlorination is one of the most important stages in the treatment of drinking water due to its effectiveness in the inactivation of pathogenic organisms. However, the reaction between chlorine and natural organic matter (NOM) generates harmful disinfection by-products (DBPs), such as trihalomethanes (THMs). In this research, drinking water quality data was collected from the distribution networks of 19 rural and semi urban systems that use water sources as: springs, surfaces, and a mixture of both, in three provinces of the Paci c slope of Costa Rica during April 2018 to September 2019. Twelve models were developed from four data sets: all water sources, spring, surface, and mixture of spring and surface waters. Linear, logarithmic, and exponential multivariate regression models were developed for each data set to predict the concentration of total trihalomethanes (TTHMs) in the distribution networks. Concentrations of TTHMs were found between < 0.20 to 91.31 µg/L, with chloroform being the dominant species accounting for 62% of TTHMs on average. Turbidity, free residual chlorine, total organic carbon (TOC), dissolved organic carbon (DOC) and ultraviolet absorbance at 254 nm (UV 254 ) showed a signi cant correlation with TTHMs. In all the data sets the linear models presented the best goodness-of-t and were moderately robust. Four models, the best of each data set, were validated with data from the same systems, and, according to the criteria of R 2 , SE, MSE and MAE, spring water and mixed spring/surface water models showed a satisfactory level of explanation of the variability of the data. Moreover, the models seem to better predict TTHMs concentrations below 30 µg/L. These models were satisfactory and could be useful for decision-making in drinking water supply systems and be considered in possible modi cations in current legislation. prediction in Costa Rica, and to the best of the authors knowledge in the Central American and Caribbean region. Furthermore, this study was focused in rural and semi urban areas, where no studies was found in the literature.In Costa Rica, 93% of the population received drinking water in 2019 (PEN and CONARE 2020). Moreover, in the same year, 19.4% of homes in rural and semi-urban areas were supplied with water by local Associations Administrators of Aqueduct and Sewerage Systems, (ASADAs in Spanish) (Sánchez-Hernández 2019). In addition, in 2016, 14.3% of the population was supplied by 24 municipalities and the rest by duly organized public companies (AyA 2016). The main water sources used are groundwater, springs, surface water and the mixture of the two latter ones, moreover, in all cases chlorine disinfection is the method used (Arellano-Hartig et al. 2020). In general, due to economic and analytical capacity limitations, monitoring of THMs is scarce, mainly at the ASADAs and municipal level. Thus, the objective of this study was to develop a series of prediction models of TTHMs in the distribution systems of rural and semi-urban areas supplied by springs, surface water and t...