Abstract Monazite–xenotime thermometry is a potentially powerful technique for understanding the evolution of Earth systems. While a rich set of experimental and empirical datasets are available for monazite–xenotime equilibria, five different thermometric calibrations yield significantly different results, making this technique difficult to apply in practice. To clarify best practices for monazite–xenotime thermometry, a compilation of published compositional data for monazite and xenotime with independently determined pressure–temperature conditions is evaluated. For each existing thermometer, we examine how closely estimated temperatures match independent empirical temperatures and consider how best to calculate monazite end-members for each thermometer. Monazite–xenotime thermometry is applied to samples from the Northern Highlands Terrane of northern Scotland, which experienced amphibolite–upper greenschist facies metamorphism and penetrative deformation during the Scandian orogeny. Thermometry data in conjunction with U–Pb dating define relatively slow regional cooling across the Scandian thrust nappes. Thermometry data closely match quartz c -axis fabric-based deformation thermometry across the structurally lower nappes, suggesting that monazite and xenotime record the timing and temperature of penetrative deformation and shearing. The data suggest that ductile deformation in the hinterland nappes of the Scandian orogen in Scotland occurred as late as 415–410 Ma.