Basanitic glasses with known dissolved C concentrations have been analyzed using infrared spectroscopy to calibrate the molar absorptivity of carbonate. C is dissolved as carbonate complexes in the glass resulting in absorption bands at 1525 and 1425 cm−1. Molar absorptivities of 283 ± 8 L/(mol·cm) were determined for both 1525 and 1425 cm−1 absorption bands. An integrated molar absorptivity of 60000 ± 1700 L/(mol·cm2) was determined using the integrated area under the doublet. These values are about 20–25% lower than those determined for tholeiitic and leucititic glasses and are intermediate between values characteristic of Na- and Ca-rich silicate glasses. Carbonate molar absorptivities for a range of basaltic glass compositions correlate with the molar ratio of Na/(Na + Ca).