Abstract Changes in reflectance and bireflectance of graptolite with increasing depths of burial were studied in four sequences of samples, two from boreholes in Poland and Sweden, and from two outcrop sections in western and northwestern Canada. Graptolites show similar optical changes (in reflectance and bireflectance), with increasing depths of burial to those of vitrinite. Maximum reflectance of low-maturity graptolites (% R o max) 1.1–1.7) increases gradually with depth of burial (at 0.14 per 100 m for the Polish samples) and at a similar but more variable rate (at 0.12–0.43 per 100 m for the Canadian samples) for high-maturity graptolites (% R o max8.0−9.0) in the chlorite zone of metamorphism. Indirect correlation between the optical properties of graptolites and those of vitrinite has been established using conodont alteration indices. In general, graptolites show higher reflectance levels than vitrinite for the same conodont indices of thermal maturity. For immature rocks (% R o vitrinite = 0.2−0.5, CAI: 1.5), graptolite maximum reflectances range from 0.6 to 1.2; for the oil generation zone % R o vitrinite 0.5−1.30, CAI: 1.5–2.5) from 1.2 to 2.2.