This paper reports on a unique evaluation-opportunity for comparing modules from two-different manufacturers that were deployed for a long-period (similar to 15 years) under similar circumstances: (a) for the same distributed, rural-power applications, (b) operating under the same climate-conditions and geographical area, (c) with devices manufactured and installed at the same time. But these modules had very different resulting observed changes in their operational and physical characteristics over their long field-exposures. The PV modules were dispersed in a tropical climate-zone (northern-region of Minas Gerais, Brazil). Visual inspections of the two-module sets concluded that one module source (SET A) had almost no encapsulant discoloration and few indications of delamination and corrosion. In contrast, the other module source (SET B) had significant yellowing/ browning, widespread-areas of delamination, and fairly extensive interconnect-corrosion. The corresponding measured changes in the electrical characteristics were: SET A having average annual loss in power of 0.4-0.5%, and SET B with 2.3-3.7%/year over their installation times. Encapsulant discoloration and delamination provided the first clues to the measured differences in module performance and are attributed to the existing climate conditions of high-ultraviolet (UV) radiation exposures and high-ambient temperatures. The bases for the performance differences of these two module sets were examined using complementary electrical, physical and chemical characterization of the module materials. The primary goal of this paper is to identify the root causes for the degradation differences (one set meeting specifications, the other not), based upon the critical module materials properties/processing and climatic exposures.