This paper reports the microbiological, geochemical, and X-Ray powder diffraction (XRD) analyses on water and/or reverse osmosis (RO) membrane cartridges in order to establish the nature of the deposits responsible for the premature blockage of RO membranes. The membrane autopsy revealed the irregularity in resin application of the membrane layers and excessive deposits on membrane cartridges. XRD phase composition obtained from Rietveld method revealed that the inorganic crystalline materials part of the deposits on the aged RO membranes mainly consisted of calcium carbonate (CaCO3) (˃ 99.00%), which was further confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric analysis. Microbial analysis indicated that microbial activity is unlikely to have played any significant role in the membrane blockage. It is concluded that the premature RO membrane blockage is mainly due to the deposition of carbonate scale and defects in membrane sealing. The mitigation strategy for carbonate scale has been discussed.