Social networking applications such as WhatsApp have been extensively used for language research; however, they have rarely been applied for language assessment purposes. To explore the efficiency of WhatsApp for assessment purposes, 30 Iranian English learners doing self- and peer-assessments on WhatsApp are studied. The changes and the reasons for the changes in their attitudes towards the two assessment types are also investigated. In a multi-phase study, the participants were trained on the new concepts of mobile-assisted self- and peer-assessments. They were also involved in the concurrent tasks of self- and peer-assessments as well as think-aloud protocols and filled out four attitude questionnaires before and after their involvement in the two assessment types. Finally, they were interviewed for the reasons of change(s) in their attitudes. The "t"-test and think-aloud results show that though the participants assigned different grades to themselves and their peers, this is not a procedural difference. The questionnaire results show that the participants generally adopted negative attitudes towards mobile-assisted assessments after being involved in them. They also gave various reasons for the change(s) in their attitudes. The results can substantially contribute to the ongoing debates on the use of alternative assessments through mobile device applications.