Over the past twenty years, the high penetration of mobile phones as a means of interpersonal communication, especially among adolescents, has facilitated access to broader social environments outside their own family. Through the extension of their social environment, teenagers are able to establish new and more extensive relationships, while facing risks that may negatively affect their socialization process. The aim of this article was to find out how computer-mediated communication helps or obstructs the creation of social capital between teenagers, and what are the consequences of its use for this age group. To achieve this, an index of social capital was developed in the study, designed to determine the positive or negative impact of certain components of mobile mediated communication in the creation of this intangible resource. Questionnaires were distributed among Spanish adolescents of secondary and high school age, from different public and private schools of Navarre. Furthermore, the study considered the adolescents' own perceptions about the incidence of the use of mobile phones in their social relationships. As reflected in the results, to identify the components of mediated communication that significantly affect social capital it is necessary to conduct an objective measurement of this resource.