Profession- and job-related social events such as mixers are viewed by organizations and individuals as incubators of interpersonal ties, as arenas in which individuals can initiate new and different contacts. Theory and evidence on network dynamics, however, suggests that such outcomes may be unlikely, because past ties constrain future contacts, and because homophily inhibits contact between different types of people. We investigate whether guests at a social mixer "mix" despite these influences. We conducted the first fine-grained analysis of the pattern of socializing dynamics at a weakly structured event, a mixer attended by about 100 business people. The results show that guests did not mix as much as might be expected in terms of new contacts. They were much more likely to encounter their pre-mixer friends, even though they overwhelmingly stated before the event that their goal was to meet new people. On the other hand, guests did mix in the sense of encountering others that were different from themselves in terms of sex, race, education and job. There was no evidence of homophily in the average encounter, although it did operate for some guests at some points in the mixer. We consider the implications of these results for organizations and individuals that seek network development, and for theories of network dynamics.