Although frequent reference is made to l'on as an alternative to on in standard grammars, judgements vary as to whether or not l'on is used at all in spoken French. This question is investigated here by treating l'on ∼ on as a sociolinguistic variable. A review of the historical, dialectal and cross-linguistic background is followed by an examination of the traditional ‘rules’ for the use of l'on, as described by Vaugelas and reconsidered by Goosse for written French. The frequency of l'on in speech, and the contexts in which it is used, are then examined in a corpus of informal interviews, and some comparisons are made with a corpus of TV news reports. (The total N of on + l'on in the two corpora is 3,549.) In general, l'on can be viewed as a ‘long form’, marking formality and comparable in some respect to l'un(e), cela, ce sont and nous -ons. But there are some individuals who use l'on even when speaking informally, especially in relative clauses opening with que.