Thermal decomposition of Fe(CO), vapours diluted in a buffer gas (Ar) was performed in a static optical cell heated up to 673 K. The nascent Fel formation in the gas phase was monitored by recording the Degenerate Four Wave Mixing (DFWM) spectrum of the a(5)D(j)-y(5)D(j') atomic transitions around 302 nm. Line-splitting phenomena, occurring upon strong saturation conditions at high laser power, were investigated on the DFWM lineshape. The role played by the absorption of exciting and signal beams in altering the relative tines intensities, predicted assuming an optically thin medium, is discussed and modelled in order to account for the experimental results.