The Yukawa–Tsuno equation, first developed in 1959, is a linear free-energy relationship in physical organic chemistry. It is a modified version of the Hammett equation that accounts for enhanced resonance effects in electrophilic reactions of para- and meta-substituted organic compounds. This equation does so by introducing a new term to the original Hammett relation that provides a measure of the extent of resonance stabilization for a reactive structure that builds up charge (positive or negative) in its transition state. The Yukawa–Tsuno equation can take the following forms:
where  and  represent the rate constants for an X-substituted and unsubstituted compound, respectively;  represents the Hammett reaction constant;  represents the Hammett substituent constant;  and  represent the Hammett substituent constants for reactions in which positive or negative charges are built up at the reactive center, respectively; and  represents the Yukawa–Tsuno parameter.