Two runs of a distributed system can be considered equivalent if they represent di erent interleavings of the same run. Formulas of the propositional temporal logic of linear time (PTL) are said to be trace consistent when they cannot distinguish between equivalent runs. Determining whether a formula is trace consistent is decidable. In this paper, we consider a closely related notion: a PTL formula is said to be inevitable, if for every system run, there is at least one equivalent run (that

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... one interleaving of that run) which satis es the formula. We show that determining whether a formula is inevitable is undecidable. However, for a subclass of formulas termed coherent (propositions in them can be a ected by either of a pair of independent operations but not both), inevitability becomes decidable.