We investigated the capacity of our system to capture CO2 by tracking the pH under photoirradiation and subsequent CO2 exposure. As shown in Figure , solutions of PBMeOH (10 mM in 2:1 MeCN/H2O) were irradiated with a 390 nm lamp, leading to a pH jump from 7 to 11.6 over 5 min. Exposure to CO2 streams resulted in a pH decrease as the hydroxide ions were converted to bicarbonate or carbonate, with the rate of capture and the terminal pH depending on the concentration of the input stream. First, under bubbling 100% CO2, a near-instantaneous decline of the pH to 6.0 was observed within 1 min, indicating nearly complete utilization of the theoretical capacity. If instead, CO2 from human exhalation (3.8% v/v) was used as the input stream, the pH decreased to 7.7 over 2 min illustrating CO2 capture was still rapid and nearly complete (hydroxide concentration decreased by 4 orders of magnitude). Interestingly, simulated direct-air capture was successful, reaching a pH of 9.7 over a 20 min period, and continuing to decrease thereafter but very slowly. We monitored the fluorenyl carbocation concentration during this process by UV-vis spectroscopy (SI Figure ), finding a gradual increase starting from the time at which air bubbling began. To our knowledge, this is the first demonstration of effective direct air capture using photogenerated hydroxide. We can estimate by charge balance that ca. 99% of hydroxide for capture was effectively utilized. Although a calibrated lamp was used for the test experiments above, the system can also be driven by natural sunlight. After exposing the photobase solution to direct sunlight (Figure ), a pH increase was observed after only 3 min, although to a slightly lower terminal value (10.8). Exposure to a 3.8% CO2/air stream again led to pH decrease to 7.0 indicating effectively complete neutralization of all hydroxide ions by reaction with CO2.