The oxidation behavior of graphite substrates with silicon carbide (SiC) and silicon nitride (Si$_3$N$_4$) coatings, prepared by using a solid-vapor reaction process, was investigated in cyclic oxidation tests. The effect of the porosity of the substrate on the oxidation behavior was also investigated. Substrates with the SiC coating showed higher weight loss than those with the Si$_3$N$_4$ coating. Oxidation of the substrates with 10 and 13 \% porosities with the Si$_3$N$_4$ coating took place at 900 $^circ$C and at 1000 $^circ$C, respectively. However, after long term cyclic oxidation tests, the substrate with 13 \% porosity with the Si$_3$N$_4$ coating was oxidized at a relatively low temperature of 800 $^circ$C, even though the weight loss was below 10 \%. In the case of the SiC coatings, detachment of the coating region or full oxidation of the substrates took place whereas in the case of the Si$_3$N$_4$ coatings, no evidence of delamination or detachment of the coating region was observed after oxidation. The oxidation was affected by the coating thickness and by the type of materials coated on the substrate, not by the porosity of the substrate itself, although the coating thickness did depend on the porosity of the substrate.