Two new members of the stolonoxide family, stolonoxides E (1) and F (2), were isolated from samples of the marine ascidian Stolonica socialis collected in Cádiz (Spain). Their structures were determined by a combination of techniques, including (+)-HRESIMS, 1D and 2D NMR spectroscopy, and comparison with published data for the structurally related stolonoxides A-D (3-6). Both compounds displayed cytotoxicity against a panel of three human tumor cell lines.