The objective of this paper is to describe a task-based project in tandem via e-mail, and to discuss the effects of motivation on task performance. In this project, a group of Irish students and a group of Spanish students are asked to carry out a series of tasks in collaboration with their tandem partners via e-mail by means of a web page especially designed for the project. Half the message is meant to be written in the student’s native language and half in the target language, and students are also encouraged to correct one another. The goal behind our research is to discuss the effects of motivation on task performance. We argue that resource directing (such as reasoning demands) and resource depleting factors (such as prior knowledge) which belong to task complexity in Robinson’s model (Robinson, 2001) are closely connected to affective variables which, as is the case with motivation, belong to task difficulty. Motivational factors like interest in the meanings to be exchanged, involvement in the decision-making process, students’ expertise in the topic, media and materials used, and the diffusion of outcomes among others have strong effects on task performance, and should therefore be considered together with complexity variables.