We investigated phase-separated self-assembled monolayers (SAMs) of alkanethiol molecules on Au(1 1 1) surfaces as model samples which have nanometer-scale domain distribution by non-contact atomic force microscopy (NC-AFM). We made SAMs composed of 1-decanethiol (CH3-(CH2)g-SH; C-10) and 1, 1 0-decanedithiol (HS-(CH2) (10)-SH; C10D). We obtained topographic, surface potential (SP) and energy dissipation images. The obtained topographic images suggested the sample surface was composed of closely packed C-10 monolayer domains and C10D domains. The SP of C-10 domains were about 100-150 mV higher than that of C10D domains. The energy dissipation on C10D domains were larger than that on C-10 domains. We discussed the origins of difference in SP and the dissipated energy on the C-10 and C10D domains. (C) 2002 Elsevier Science B.V. All rights reserved.