On plasma-enhanced chemical vapor deposition of aluminium oxide films using trimethylaluminum (TMA) and N2O gases, optical emission spectroscopy (OES) analysis was performed to identify the ions and the radicals present in the glow discharge. A floating double probe was used to measure the plasma electron temperature and ion density. OES analysis of N2O gas-He plasma showed that N2O gas was dissociated into the form of NO and N-2. The He plasma was cooled by adding N2O and TMA. TMA was more effective in cooling the electron temperature than N2O gas. The radical concentration did not change with the mole fraction of the reative gases because of the cooling effect of the reactive gases, which affects the deposition rate. From the results of the floating double probe, it was found that r.f. power affects the plasma density rather than the mean electron energy. With an increase of r.f. power from 5 to 30 W in the He plasma, the plasma denisty of He plasma varied from 2.5 x 10(15) to 8 x 10(15) m(-3) and the mean electron energy was in the range of 4.1-3.5 eV.