Tradable permits and carbon taxes are two market-based instruments commonly considered by policymakers to regulate pollutions. While a tax is fixed, predetermined by authorities, the uncertain permits price is driven by market dynamics, fluctuating with the prices of natural gas and electricity. Both instruments offer firms different incentives for adopting clean technologies. This paper explores the optimal investment timing when a coal-fired plant owner considers introducing clean technologies in face of these two policies using a real options approach. We find that tradable permits could effectively trigger adopting clean technologies at a considerably lower level of carbon price relative to a tax policy. Higher levels of volatility in permit prices are likely to induce suppliers to take early actions to hedge against carbon risks. Thus, offset and other price control mechanisms, which are designed to reduce permit prices or to suppress prices volatility, are likely to delay clean technology investments. doi: 10.5547/ISSN0195-6574-EJ-Vol32-No3-6