The adsorption and the thermal decomposition processes of C2H2 on Pt(111) have been investigated by the use of scanning tunneling microscopy. For a high C2H2 coverage, a (2 x 2) structure is observed locally after the C2H2-covered Pt(111) surface is heated to 120 K. By heating to 220 K, the sample surface is covered with the (2 x 2) structure separated with brighter lines. After heating up to 370 K (2 x 2) domains related to ethylidyne species appear with larger and brighter protrusions. For a low C2H2 coverage, adsorbates are located randomly and no ordered islands are observed after heating to 120 K. In contrast, heating to 220 K leads to the formation of (2 x 2) islands. This indicates that the adsorbate-adsorbate interaction in the (2 x 2) structure observed by 120 K heating is different from that in the (2 x 2) structure observed by 220 K heating; the former is repulsive, but the latter attractive. Therefore, it is suggested that different surface species are formed by the decomposition of C2H2 at 220 K. The detail of the decomposition processes of C2H2 and the relation between the adsorbate-adsorbate interaction and the surface species are discussed. The results are also reported measured by heating up to higher temperatures above 400 K. (C) 2002 Elsevier Science B.V. All rights reserved.