Abstract: In this paper, a "ratio method" of quantitative analysis by infrared spectrophotomerry was reported. This method was applied to the determination of small amounts of Camptothecin (Ⅰ) which was present as a contaminant in synthetic camptothecin hemi-ketal (Ⅱ). Compound Ⅱ showed an absorption band of lactam carbonyl group in the IR spectrum while compound Ⅰ had in addition a δ-lactone absorption band. Because of the similarity in structure and the closeness of the molecular weights of these. two compounds, it was derived from Beer's law that the ratio (R)of the δ-lactone band to that of the lactam band should be proportional to the amount of compound Ⅰ present in compound Ⅱ. Experimental results confirmed this and the plot of R vs C_I (concentration of compound Ⅰ) gave a straight line passing through the origin. Pressed KBr disks were used for the analyses. The samples need not be weighed. This method is simple and the results are reproducible to better than ±2%.