In blended learning, the internet acts as an instrument to complement traditional forms of instruction, in the belief that the incorporation of new information and communication technologies may lead to more efficient and effective education. This paper presents a study carried out in the University of Granada, during the first year of undergraduate courses, which considered a total of 1,128 students organized into 17 groups during the academic year 2009-2010 and focused on the students' voluntary use of online learning activities. The results show that the students' participation in these activities and the number of tasks completed both had a positive effect on the students' final marks. The time employed in carrying out online tasks did not influence the results achieved but the marks obtained in such activities were a significant factor. In addition, the students' respective background, rate of class attendance and interest in the subject were explanatory variables of the outcome.