Investigations into the use of private speech by adult English foreign language (EFL) learners in regulating their mental activities have been an interesting area of research with a sociocultural framework. Following this line of research, 30 advanced adult EFL learners were selected via the administration of Oxford quick placement test and took a test of solving challenging English riddles while their voices were being recorded. Later, instances of the produced private speech were analyzed in terms of form, content, and function. It was demonstrated that private speech with its different forms, contents, and functions plays a very crucial role in cognitive regulation of EFL learners which has important implications for the context of language learning classrooms. In addition, participants seemed to produce qualitatively different kinds of L2 private speech, which brought us to the conclusion that it would be necessary to consider quality and not just quantity in studying psycholinguistic concepts, such as cognitive regulation and private speech.