Oil and gas reserves and asset decommissioning are the most significant cash flow indicators of the oil and gas industry. Investors use disclosures by oil and gas firms to estimate future cash flows. Thus, this study examines impacts of oil and gas reserve disclosures and disclosures of decommissioning costs of oil and gas assets on the financial performance and value of listed oil and gas companies in the UK. We survey data from 52 listed firms. We conclude that whilst mandatory and voluntary reserve disclosures negatively impact firms' performance, they positively impact value. Mandatory decommissioning disclosures are positively related to performance and value. Voluntary decommissioning disclosures negatively correlate with firms' value, but positively relate with firms' performance. These findings contribute to the debate around the usefulness and impacts of oil and gas reserves disclosures and disclosures of decommissioning costs of oil and gas assets on companies values and performance.