Two oxygens of which δ(18)O is +28.83 and -79.57‰, respectively, were converted into carbon dioxide by means of both an external heating method and a Daylight's method where a graphite rod was heated by concentration of light from a slide projecter lamp on the graphite rod, and the δ(18)O of carbon dioxides obtained in the two methods were compared to each other. Except for data affected by memory of the previous sample, the averages in the external heating method were +28.85 and -79.56‰, respectively, and quite consistent with the averages in the Daylight's method, that were +28.81 and -79.67‰, respectively. This concludes that no oxygen isotope exchange occurs between the sample gas and the quartz glass of reaction tube in the external heating method. There is no significant difference in the results in four externally heated carbon furnaces, whereas the memory of previous samples was
apparently observed within each furnaces. This memory was larger in the furnaces containing a used graphite rod (Nos. 3 and 4) than those containing new one (Nos. 1 and 2), and also more significant for smaller samples (Run 4 in No. 3). However, if the reaction tube was baked out at a higher temperature (750℃) than the reaction temperature (700℃) before reaction, the memory was mostly eliminated (Run 8 in No.3).