Repeated heterogeneous nucleation of a single, unchanging aqueous sample of water in a container coated by a self-assembled monolayer (SAM) is studied using our automated lag-time apparatus showing that coating the walls of the sample container with the SAM decreases the average supercooled temperature (DeltaT(50%)) to colder temperatures, indicating that nucleation in an unmodified glass container takes place on the container walls in the absence of a substrate or impurities. Adding an AgI crystal increases DeltaT(50%) to the same warm temperature, whether the container is SAM-coated or not, suggesting that nucleation takes place on the surface of the added crystal. (C) 2004 Elsevier B.V. All rights reserved.