1] Temperatures derived from Mg/Ca ratios of the calcite tests of planktonic foraminifera are distorted when samples are partially dissolved, and methods are required to quantify this source of inaccuracy. Here we compare a dissolution index (XDX), based on X‐ray computed tomography scans, to Mg/Ca of four species of foraminifera (G. ruber (white), G. sacculifer (without sac), N. dutertrei, and P. obliquiloculata) from core top sediments from the tropical Pacific, Atlantic, and western Indian Ocean. Deepwater calcite saturation values (D[CO 3 2− ]) at the sites ranged from 55 to −23 mmol/kg. An estimate of DMg/Ca (reduction in Mg/Ca due to dissolution) was made for each sample. DMg/Ca decreased linearly from deepwater D[CO 3 2− ] values of between 10 (±4) and 15 (±5) mmol/kg. These values are minimum estimates of the threshold below which Mg/Ca is affected by dissolution, as they are limited by assumptions made in calculating DMg/Ca. Sensitivity of Mg/Ca to D[CO 3 2− ] was greatest for G. ruber, where Mg/Ca decreased by 0.102 (±0.036) mmol/mol per mmol/kg. Sensitivity was similar for G. sacculifer (0.047 ± 0.015 mmol/mol per mmol/kg), N. dutertrei (0.037 ± 0.010 mmol/mol per mmol/kg), and P. obliquiloculata (0.040 ± 0.008 mmol/mol per mmol/kg). Sensitivity was similar at all sites for each species, excepting an apparently greater response for N. dutertrei from the Caribbean compared to other sites. Calibrations between XDX and DMg/Ca provide a means to estimate dissolution bias on Mg/Ca. Poor correlation between XDX and d 18 O suggests that, for the small sample size typical for analysis, variability in initial d 18 O overwhelms dissolution effects.