Anthropogenic inputs of nutrients to coastal waters fuel the excessive production of phytoplankton, a process known as eutrophication, and the resultant export of organic matter stimulates microbial respiration in the subsurface waters. In addition to the decline in oxygen levels, the carbon dioxide produced during microbial consumption of organic matter increases acidity. Water mass mixing with different ratios of dissolved inorganic carbon (DIC) to total alkalinity (TA) from the subsurface water also modulate acidity to a large extent. Here, we assess the synergistic impacts of eutrophication and water mass mixing on acidity in a large subtropical estuary system, the Pearl River Estuary (PRE), featured by large freshwater runoff and heavy nutrient loadings from the Pearl River and extensive seasonal hypoxia in summertime. Using data collected in the PRE and its adjacent waters during a cruise under the Ocean-HK project conducted in July 2017, a semi-analytical diagnostic approach was applied to resolve pH dynamics based on a validated multiple-endmember water mass mixing model. We show that the acidification of subsurface waters was exacerbated with the development of hypoxia (dissolved oxygen, DO, less than 2 mg L-1 or ~63 umol kg-1) as expected and the drawdown of pH could be up to -0.31+/-0.05 units in the hypoxic zone. pH deviations from the modelled pH-DO line due to water mass mixing varied from -0.08 to 0.12 units, depending on the DIC/TA ratios and fractions of freshwater and surface water that advected into the subsurface layer. Using model simulations, we will also present how eutrophication, rise in anthropogenic CO2 and temperature as well as community respiration rate would modulate the pH, in the future scenarios.