This paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration, namely sericitization, chloritization, and greisenization. The samples were taken from three Paleozoic granites: the St. Nectaire granite (Massif Central, France), the Carnmenellis granite (Cornwall, England), and the Skiddaw granite (Lake District, England). Fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340°C and salinities from 3 to 18 wt% NaCl equivalent. Possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy (BSE-SEM), electron microprobe, laser Raman spectroscopy and laser ablation—inductively coupled plasma—mass spectrometry (LA-ICP-MS) include cationic substitutions, monoclinic to hexagonal structure transition accompanied by chemical exchanges, selective Th removal, dissolution-reprecipitation, and dissolution with replacement by a different mineral. These results show that, despite the compositional and crystallographic simplicity of monazite, it exhibits varied chemical responses to different mineral-fluid interactions. Elemental and isotopic measurements by LA-ICP-MS for the unaltered monazites yield statistically significant 232Th- 208Pb and 238U- 206Pb magmatic crystallization ages. In some cases it was possible to give a reasonable estimate of the age of mineral-fluid interaction using the altered parts of the monazites or newly precipitated crystals. For other minerals, 232Th- 208Pb and 238U- 206Pb systematics were strongly disturbed by variable inputs and/or depletions of U, Th, and Pb. The data suggest that monazite-like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies, especially if the fluids are oxidizing or tend to dissolve monazite.