Spectacular alteration of monazite by diagenetic/hydrothermal brines is well documented in some Proterozoic sedimentary basins in close relationship with high-grade uranium (U) deposits. Hence, monazite has been proposed as a viable source for some U deposits. However, monazite alteration remains enigmatic with regard to its high stability in relatively low temperature hydrothermal conditions. Here, the results of batch experiments in which 10 mg of natural monazite grains were reacted with 15 mL of Na-Ca-Cl (6 molal Cl) solutions as well as in pure water at 150 degrees C and saturated vapor pressure (p(sat)) for one and six months are reported. The influence of pH (pH = 1, 3, 7) and relative molar proportions of Na and Ca (Na/(Na + Ca) = 0, 0.5, 1), were tested. Discrete alteration features (etch pits and roughened surfaces) appear in a minority of the one month experiments and are more developed in the six months experiments, especially at pH = 1 and 3. Although spectacular alteration of monazite, as seen around U deposits, could not be reproduced here, this study shows that monazite is unstable in the presence of fluids analogous to acidic deep basinal brines.