We extend the WACC approach to a tax system having a firm income tax and a personal income tax of the investor as well. We use an artificial tax system incorporating most of the G-7 national tax codes as for example the classical or the imputation systems.

A simple counterexample shows the the widely used WACC approach to value leverage firms developed by Miles and Ezzell can create an arbitrage opportunity. The only consequence to be drawn is that their WACC approach cannot be applied under the circumstances assumed by Miles and Ezzell.

Calculating the cost of capital has been always a key issue in financial management. One way to calculate cost of capital is using the weighted average cost of capital (WACC) formula which has been presented by Modigliani and Miller. Recently, validity and correctness of WACC have been...