The retention mechanism of solutes under gradient conditions has been studied. Separation of a mixture of seven aromatic compounds in the two binary mobile phase, water/methanol and water/acetonitrile, was considered as an example. Retention factors were experimentally correlated by mobile phase composition. In this work, gradient-deviation time was newly introduced to compensate for ideal steep band along a column and experimentally determined by a linear equation form. An analytical expression in terms of the calculated retention factor and peak width was presented to predict the elution profile under gradient conditions. The calculated elution profile considered by the gradient-deviation time was closer to experimental data, and this mathematical model showed the feasibility of a predictive tool under gradient conditions.