The article focuses on the features of the formation of the lexis of positive moral-ethical evaluation in the Spanish language of the 13–15th centuries. The research is based on the material of the Mediaeval Spanish adjective virtuoso (virtuous), which in the studied period belonged to the core lexis of positive moral-ethical evaluation. This is explained by the role the Church played in the life of the mediaeval society, in the formation of ethical norms and a particular system of values. This is confirmed by the fact that the subject of sin and virtue occupied a significant place in the analysed texts. Mediaeval Spanish written monuments of the 13–15th centuries both religious and secular in nature serve as the source of the research material. The written monuments are contained in the Corpus del español electronic corpus. The use of corpus data made it possible to study those lexico-semantic and thematic relations that were inherent in the lexeme virtuoso in the period in question. A significant part of the analysed contexts, the main purpose of which was to name and characterize Christian virtues, along with the virtuoso lexeme, contains chains of lexemes of positive moral-ethical evaluation, which were the names of certain virtues: santo, poderoso, vergonzoso, honesto, etc. Positive moral-ethical evaluation was especially clearly manifested when the lexeme entered into antonymic relations with adjectives denoting various sins and vices: malo, vicioso, malicioso, bravo, etc. Subsequently, the lexical meaning of the adjective virtuoso, characteristic of the above-mentioned contexts, becomes widespread outside the religious sphere, and over time it becomes the main meaning at the systemic level.