In this work, we investigate using carbon dioxide (CO2) as an inexpensive reaction medium during the photocatalytic oxidative coupling of primary amines to the corresponding imines. As a model system, we study the reaction of benzylamines in acetonitrile over a potassium poly(heptazine imide) photocatalyst (K-PHI). We use CO2 or argon (Ar) to saturate the reaction solution and the reactor headspace, and blue light (461 nm) to drive the reaction. When compared to the more expensive Ar, CO2 boosts the conversion rate by creating a reaction intermediate with better electron donor properties. We also validate the influence of water content on the imine formation efficiency, and we observe higher catalytic activity of K-PHI, compared to a standard mesoporous graphitic carbon nitride (mpg-CN).