We examine whether voucher programme regulation reduces private school specialisation in the US. We employ school-and-year fixed-effects regression and examine individual private schools in Washington, DC, Indiana, and Louisiana as they transition into voucher programme environments. We leverage the Private School Universe Survey to observe how schools self-identify before and after switching into voucher environments. We find that upon switching into school voucher environments, private schools in all 3 locations are more likely to identify as less specialised than they were prior to entering the programmes. We find suggestive evidence that the homogenising effects may be stronger in more heavily regulated voucher programme environments, and that those schools in more lightly regulated environments are more likely to continue highlighting their specialised approach to education. These findings are examined within an institutional theory framework to understand the potential homogenising effect of regulations on the diversity of the private school market.