A procedure for spectrophotometric determination of traces of phosphorus(P) in high-purity trichlorosilane(TCS) is proposed using an adsorptive separation. $PCl_3$, which is a dominant P impurity within TCS, is first oxidized by oxygen to a stable form as $POCl_3$. $AlCl_3$ is selected as an adsorbent which forms a thermally stable complex with $POCl_3$ in TCS and can be well dissolved in aqueous ethanol solution. The proposed adsorptive separation method is free from the formation of silica gel and gas bubbles during the colorimetric analysis of TCS. The method reveals that the P concentration in a semiconductor-grade TCS is 5.32 ${mi}g/l$ within the standard deviation of ${pm}$ 17%. On the other hand, the P concentration of the purified TCS which is separated from the $AlCl_3$${cdot}$$POCl_3$ complex is reduced to be less than 0.15 ${mi}g/l$, showing the efficient applicability of $AlCl_3$ to the wet chemical analysis. The proposed method is also tested to verify the effectiveness of other well-known adsorbents.