Decomposition conditions of methane hydrates in sediments were measured during formation-decomposition cycles. As test sediments, we used silica sand, sandstone, and clays (kaoline and bentonite), which are typical natural materials known as hydrate bearing sediments, and the range of samples cover a range of water saturating abilities. To better understand the results, we also used uniformly sized glass beads. Pore effects on decomposition of these materials were investigated by analyzing the pore-space distributions of the materials and by varying the initial water content of the samples. The results obtained for sand and sandstone samples indicated that the final decomposition temperatures were shifted lower than those for bulk hydrates at the same pressure. Temperature shifts were more negative for smaller initial water contents with the maximum shift being approximately -0.5 K. The results were consistent with those measured for glass beads with nearly the same particle size. For kaoline clays, the shift was at most -1.5 K. We conclude that the decomposition conditions are mainly affected by the pore sizes. The surface textures and mineral components had less influence on the results. We confirmed that glass beads mimic the effect of sediments for sand, sandstone, and kaoline clays, which have little to no swelling when put in contact with water. On the other hand, for bentonite particles, the results indicated that methane hydrates formed not only between the particles but also in the interlayers. A thermodynamic promoting effect was found for dilute bentonite solutions, although the positive decomposition-temperature shift was at most +0.5 K.