Governments and managers have generally conducted inadequatecost-benefit analysis (CBA) by failing to incorporate environmentalimpact costs. This is not a straightforward task, owingto the intangibility, in monetary terms, of most environmentalgoods and services. By means of a theoretical example, we presenta practical approach to correct cost-benefit analysis for environmentalexternalities that do not need natural resourcevaluations. The theoretical example is based on ventures thatpotentially pollute river waters. However, we think that themethodological approach presented here may be used to internalize(in monetary terms) the costs of negative external impactson the environment, which may result from any type of ventureor project.