A multiperiod mixed-integer nonlinear programming (MINLP) model is presented for the planning of an optimal investment policy for the gradual retrofit of chemical plants over a specified time frame. This approach can be applied when the available funds are insufficient to perform the optimal reconstruction in one step. Through gradual improvements, the process progressively approaches its optimum with considerably lower requirements for new capital, while still exhibiting an attractive increase in economic performance. In the proposed multiperiod problem for gradual retrofit, certain amounts of capital enter into the time periods for the gradual increase of equipment in order to improve the performance of the process, e.g., to lead to an increase in conversion or energy recovery. This capital may arise from internal sources within the company or can be borrowed. The financial benefits resulting from the improvements at the end of the time period can be reinvested for the additional reconstruction over the next development phases. In this way, self-sustained capital generation for future reconstruction is achieved, and any additional funds required are minimized. The objective function of the proposed problem is formulated as the net present worth of the gradually retrofitted system. This enables several techno-economic analyses to be performed. The proposed multiperiod investment strategies are illustrated by an investment planning example for the retrofit of a heat-exchanger network.