Silica gel treated with thionyl chloride was found to be an effective as well as highly selective catalyst for thioacetalization of aldehydes. With the use of this catalyst 1,3-dithioranes and 1,3-dithianes were obtained in excellent yields from various aldehydes. Under the same conditions ketones were similarly but more slowly thioketalized. This difference in reactivity between aldehydes and ketones was successfully utilized for the thioacetalization of aldehydes in the presence of ketones and also for the chemoselective conversion of keto aldehydes into the corresponding dithioacetals with the keto group remaining intact.

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