The purpose of this thesis is to examine and compare Thai and UK income tax laws to establish how they cause conflict between equity of income redistribution and efficiency of taxation. This thesis also aims to validate theories that optimal tax structures and efficient tax legislation and administration can resolve the conflict. Chapter One reviews concepts of equity and efficiency. Research in the components of income tax law to establish optimal tax structures offers a novel way to resolve the conflict. Chapter Two concerns a progressive rate structure. At present, there are few low marginal tax rates, resulting from revenue targets, politics, tax avoidance, and few top-rate taxpayers. They achieve only some redistribution, but promote efficiency. A system of two tax-rate schedules is proposed, i.e. one for low incomes and another for high incomes. Chapter Three involves a tax base. Tax reliefs narrow the base, affecting equity and efficiency. An optimal base is therefore proposed, with a broad base, adjustment of personal allowances to cover costs of living and inflation, a phase-out of allowances, the abolition of standard deduction of expenses, and criteria for abolishing certain tax expenditures. Chapter Four considers the tax treatment of married couples. The Thai joint-taxation system needs to increase efficiency yet maintain equity. The UK individual-taxation system needs to reduce inequity while maintaining efficiency. To strike a balance between equity and efficiency, the integration of joint and individual taxation, changes in the structures of tax rates and reliefs, and prevention of tax avoidance are proposed. Chapter Five analyzes legislative process, primary and secondary legislation, judicial interpretation, judicial and statutory anti-avoidance rules, administrative practice and methods of tax collection.