Shape changes of Ag and Au nanoparticles supported on single crystal reconstructed SrTiO 3 (001) and (111) substrates were investigated using scanning tunneling microscopy. Both metals nucleate as multiply twinned particles (MTPs) and transform into face-centered-cubic single crystals (SCs) beyond a critical volume. On SrTiO 3 (001)- c(4 × 2) the critical volumes are measured as 141 ± 51 nm 3 for Ag and 107 ± 23 nm 3 for Au, whereas on SrTiO 3 (111)–(4 × 4)+(6 × 6) the critical volumes are 53 ± 26 nm 3 for Ag and 26 ± 40 nm 3 for Au. A much larger transition volume was observed on SrTiO 3 (001)–(2 × 1), where Ag remains as MTPs up to 3400 nm 3 , while Au nucleates as atomic monolayers instead of MTPs. This work demonstrates the significant impact of small variations of the surface structure of the substrate on the MTP–SC transition volume.