Organic matter is classified in groups, and total organic carbon (TOC) is the sum of dissolved organic carbon (DOC) and particulate organic carbon (POC). DOC composes most of the TOC and therefore it is our main focus. Puerto Rico has been showing an increase in urbanization, especially in the San Juan Metropolitan Area, which is decreasing the extensions of riparian zones. This in turn reduces the natural inputs of allochthonous organic matter to rivers and are being replaced by organic loads from wastewater and urban runoff. The purpose of this project is to characterize the quality of organic carbon in an urban tropical river. Studies have shown that organic carbon originating from sewage discharge is more labile than that of natural sources (Paul & Meyer 2001). We hypothesized that if the main source of organic matter in an urban tropical river is from riparian zones located in the upper parts of the basin, then its biodegradability will decrease downstream as microbial and physical processes decrease its lability.