Online sustainability communication attracts a considerable attention in tourism research. This study focuses on sustainability communication in official destination websites for informing and motivating visitors to adopt sustainable practices and behaviors. To advance previous research in this area, it adopts a web content mining approach to assess the characteristics of online contents of a sample of 20 Italian cultural destinations. The main dimensions and typologies of sustainability-oriented practices in tourism are used as basis to develop a text classifier for the automated content analysis. A total of 2.975 web pages from official city websites and official tourism promotion websites of the destinations is analyzed through this approach to investigate the online contents relating to the environmental, economic, socio-cultural and general dimensions sustainability as well as their emotional appeal. The analysis reveals that about 15.8% of total online texts contains information to promote sustainability-oriented behaviors at the destination. It indicates that the communication is more specifically focused on environmental/economic/socio-cultural practices than generically referring to sustainable/responsible tourism. However, in line with previous research, it highlights that destinations do not sufficiently balance these pillars in their online communication. Further, the websites’ texts scarcely leverage the persuasive potential of affective appealing messages. The implications of this automated approach for improving the design of online sustainability-related contents are also discussed.