(p, rho T) Measurements and visual observations of the meniscus for isobutane were carried out carefully in the critical region over the range of temperatures: -15 mK <= (T - T-c) <= 35 mK, and of densities: -7.5 kg (.) m(-3) <= (rho - rho(c)) <= 7.5 kg (.) m(-3) by a metal-bellows volumometer with an optical cell. Vapor pressures were also measured at T = (310, 405, 406, 407, and 407.5) K. The critical point of T, and rho(c) was determined by the image analysis of the critical opalescence which is proposed in this study. The critical pressure p(c) was determined to be the pressure measurement at the critical point. Comparisons of the critical parameters with values given in the literature are presented. (c) 2006 Elsevier Ltd. All rights reserved.