This study describes the occurrence of a molecular interference affecting the determination of phosphorus by the molecular absorption of phosphorus monoxide (PO) at 213.647 nm using high-resolution continuum source graphite furnace molecular absorption spectrometry with direct solid sample analysis (HR CS SS-GF MAS). The study shows how the observation of the background signal provided by the software, which typically goes unnoticed as it is not well understood what is represented by it, provides valuable information for method development. In this case, the background signal hints at the formation of a concomitant polyatomic molecule in all solid samples investigated that is more volatile than PO and that can cause disturbances in the absorption of the target molecule. This information is paramount to properly optimize the temperature program. The most effective modifier to achieve quantitative PO molecule formation and better thermal stability consisted of a mixture of Au and Ca. The pyrolysis temperature was set at 1500 °C to minimize the influence of the interference mentioned above. The vaporization temperature was set at 2200 °C. Following this procedure, certified reference materials of leaf samples (NIST 1547, NIST 1570a, and NIST 1573a) could be accurately analyzed simply using external calibration with P aqueous solutions. Macauba and eucalyptus real samples were also analyzed, showing P concentrations of 455 ± 39 and 400 ± 35 μg g⁻¹, respectively. The limit of detection of the method was found to be 0.078 μg (n = 5), which translates into 78 μg g⁻¹ when using one mg of the sample, which seems appropriate for this type of sample.