In this research, the tool-chip interface temperature (T-CTI), the tool temperature (T-T) and the average surface roughness (R-a) were measured experimentally during the turning of AISI 4140 alloy steel with TiAlN-TiN, PVD-coated, WNVG 080404-IC907 tungsten carbide inserts using an IR pyrometer technique, a K-type thermocouple and a portable surface-roughness measurement device, respectively. The workpiece material was heat treated by an induction-hardening process and hardened up to a value of 50 HRC. The Taguchi method L18 (21 x 37) was used for the determination of the optimum control factors. The depth of cut, the cutting speed and the feed rate were taken as control factors. The analysis of variance was applied in order to determine the effects of the control factors on the tool-chip interface temperature, the tool temperature and the surface roughness. The optimum combinations of the control factors for T-CTI, T-T and R-a were determined as a(2)v(1)f(3), a(1)v(3)f(2) and a(2)v(3)f(1), respectively. Second-order predictive models were developed with a linear-regression analysis, and the coefficients of correlation for T-CTI, T-T and R-a were calculated as R-2 = 92.8, R-2 = 68.1 and R-2 = 82.6, respectively.