The usual thermodynamic evaluation, based solely on the Standard Gibbs Energy of reaction, does not take into account the permissible ranges of concentrations of metabolites, and it faces further difficulties when, instead of isolated reactions, we are examining whole pathways. For pathways, we seek not only to decide whether they are feasible but also to pinpoint the pathway segment that causes any thermodynamic difficulties. We define a set of scaled quantities which reformulate the thermodynamic-feasibility problem for the whole pathway. We present an algorithm which analyzes individual reactions and selective construction of larger subpathways and uncovers localized and distributed thermodynamic bottlenecks of the biotransformation. This type of thermodynamic treatment contributes to the effort to include more physical, chemical, and biological factors in the computer-aided analysis of metabolic pathways.