The SEN2SOC mobile application is intended for Santander citizens and visitors, and provides a rich set of services to mobile device users (using either smartphones or tablets). The services offered by the SEN2SOC mobile application include: real-time environmental monitoring of the city of Santander derived from SmartSantander sensors’ measurements; route recommendations based on sensor measurement information; notification mechanism (alerts) on extreme environmental conditions; suggestion of areas and points of interest to city visitors; social media activity in Santander areas; and city-relevant trends derived from social media content analysis. Furthermore, the SEN2SOC mobile application implements a mechanism that allows users to express their perception on the present environmental parameters of their current location, thus in a way “validating” sensor measurements. In essence, our plan is to combine, compare, and correlate sensor quantitative information (SmartSantander sensor data) and human qualitative information (“users-as-sensors”) with regard to environmental conditions. Last but not least, mobile application users can react to issued alerts about extreme environmental conditions by means of sharing those alert messages on their social media account (Facebook/Twitter).