Two new norsesterterpene 1,2-dioxanes, mycaperoxides A (2) and B (3), have been isolated from a Thai sponge of the genus Mycale. Their relative structures were determined by X-ray and spectroscopic methods and their absolute configurations assigned by applying the Kusumi and Kakisawa modification of Mosher's method. Both compounds showed significant cytotoxicity and in vitro antiviral activity.