Abstract: An optical absorption calorimeter performs absorbance measurements at low cryogenic temperatures, such as above 0K to 5K (e.g. near liquid helium temperature), using high-resolution thermometry with SQUID readout to probe optical absorption to better than 1 ppb. This improved sensitivity yields improved performance in calorimetric absorption spectroscopy by lowering the required excitation power, improving the spectral resolution, and opening up the full spectrum, from near-IR to near-UV and beyond for analysis.