Measurements of axial temperature and electron density have been carried out for Ar-H₂ dc transferred arc-plasma columns under varying ambient-pressure conditions (100-760 torr). Optical emission spectroscopy has been used as the diagnostic tool. Temperature was determined by the relative intensity method using atomic hydrogen lines. The electron-density measurements were obtained from the Stark width of the H_b (486.13 nm) line. Possible deviation from local thermodynamic equilibrium (LTE) has been studied by using H_b transitions and by measuring the population factor using ArI (696.5 nm) transition. The nonideality along the plasma column has been studied using the electron-density data. The plasma approaches weakly nonideal conditions near substrate. The maximum deviation from LTE conditions also arises near the transferred anode substrate. With an increase in pressure, the volume of plasma approaches LTE.