Specifications of requirements for new software systems can be revised, refined or completed in reference to specifications of requirements for existing similar systems. Although realized as a form of analogical problem solving, specification by reuse is not adequately supported by available computational models for detecting analogies. This is chiefly due to the following reasons: (1) It is assumed that specifications are expressed according to the same specification model and in a uniform representation scheme. (2) Additional information is needed for the detection of analogies, which is not contained in the specifications. (3) Performance scales poorly with the complexity of specifications. This paper presents a computational model for detecting analogies, which addresses these issues to a certain extent. The application of the model in the specification of requirements by analogical reuse is demonstrated through an example, and its sensitivity to the representation of specifications is discussed. Finally, the results of a preliminary empirical evaluation of the model are reported.