To encourage a step change in private finance for low-carbon investment, we examine the potential of taxing institutional investment funds (investment emissions intensity taxation, IEIT) if the emissions intensity of their portfolios exceeds specified thresholds. We examine how IEIT would impacts assets’ relative expected rates of return, using a hypothetical diversified portfolio comprising the constituents of a leading market index. IEIT exerts a price influence on the flow of corporate capital expenditures financed either by capital-raising or by corporate retained earnings, encouraging both standalone low-carbon projects and more energy-efficient replacement of process-embedded fixed assets. IEIT must be a forward-looking tax with a pre-specified profile of reducing emissions intensity threshold levels to avoid potential offshoring of investment due to its significant impact on the cost of capital for high emissions intensity companies. IEIT challenges regulated investors to match their net zero declarations with actions, and acts as a policy backstop if emissions trading prices and coverage remain too low, or in case climate risk disclosure and investment taxonomy policies alone do not shift investment behaviours sufficiently.