This paper analyzes the substitution relations between the main inputs used in soybean production in Brazil, through the esti-mates of Allen and Morishima substitution elasticities. The theoretical approach used is the production/cost duality. The data was obtained from field research in the five main production states in the country. Chemicals, capital, land, labor and other costs were the variables under study. The estimated cross-price elasticities pointed to complementa-ry relations between labor and capital. The Allen partial substitution elasticities showed substitution between most of the production factors, but a strong complementarity relation was found between capital and labor. In the Morishima elasticity of substitution concept capital and la-bor were found to be complements when the price of capital varies and substitutes when the price of labor varies.