Involuntary mental time travel (MTT) is spontaneously reliving past events or envisioning future scenarios without conscious effort. We explored the phenomenological characteristics and contents of self-caught and probe-caught spontaneous thoughts, focusing on involuntary MTTs. These paradigms differ in the meta-awareness they demand, which may affect the nature of the captured thoughts, especially under attentional load. During a vigilance task with different attentional loads, participants reported their thoughts as they realized them (self-caught) or when the task prompted them (probe-caught). They then completed questionnaires regarding their thoughts' phenomenological characteristics. We predict that self-caught thoughts will have a higher proportion of involuntary MTTs, marked by episodic and self-related content. Under high attentional load, involuntary MTTs are expected to comprise a larger proportion of reported thoughts in both paradigms. Investigating the characteristics of spontaneous thoughts and their modulation by attentional load contributes to a deeper understanding of the metacognitive processes underlying involuntary MTTs.