<p>The effect of different acidic conditions on the fracture parameters of marble including fracture toughness of the modes I, II, and I–II, load–displacement curves, and crack propagation angle, was investigated. The specimens were treated based on the different acidic conditions in seen cycles of 2, 4, 6, 8, 10, 12, and 24 h. Sulfuric acid chemical solution at various pH levels of 2, 4, and 5 was used to simulate the different acidic environments. To examine the marble specimens more closely, microscopic studies including thin section and scanning electron microscope images, were performed before and after the treatment of specimens. Finally, 90 rock fracture toughness tests were conducted to investigate the rock fracture behavior. The results showed that the principal factor in the structure variation of marble specimens is the formation of the intergranular and to some extent intragranular microcracks due to the effect of different acidity levels. Rock fracture toughness under the influence of acid exhibited a decreasing trend, particularly at the lowest pH level (i.e., pH 2). Modes I, II, and I–II fracture toughness values and the corresponding crack propagation angles under all different acidic conditions were compared to the predictions by various mixed-mode I–II fracture criteria. It was found that the extended maximum tangential strain criterion better predicts the experimental data.</p>