Echinoids have high-magnesium (Mg) calcite endoskeletons that may be vulnerable to CO 2 -driven ocean acidification. Amalgamated data for echinoid species from a range of environments and life-history stages allowed characterization of the factors controlling Mg content in their skeletons. Published measurements of Mg in calcite (N = 261), supplemented by new X-ray diffractometry data (N = 382), produced a database including 8 orders, 23 families and 73 species (~7% of the ~1000 known extant species), spanning latitudes 77°S to 72°N, and including 9 skeletal elements or life stages. Mean (± SD) skeletal carbonate mineralogy in the Echinoidea is 7.5 ± 3.23 wt% MgCO 3 in calcite (range: 1.5−16.4 wt%, N = 643). Variation in Mg within individuals was small (SD = 0.4−0.9 wt% MgCO 3 ). We found significant differences among skeletal elements: jaw demi-pyramids were the highest in Mg, whereas tests, teeth and spines were intermediate in Mg, but generally higher than larvae. Higher taxa have consistent mineralogical patterns, with orders in particular showing Mg related to first appearance in the fossil record. Latitude was a good proxy for sea-surface temperature (SST), although incorporating SST where available produced a slightly better model. Mg content varied with latitude; higher Mg content in warmer waters may reflect increased metabolic and growth rates. Although the skeletons of some adult urchins may be partially resistant to ocean acidification, larvae and some species may prove to be vulnerable to lowered pH, resulting in ecosystem changes in coastal marine environments.