The paper presents the results of a systematic study of the oxygen and carbon isotopes in the walls of the hypogene karst conduits in the Crimean Piedmont, including conduits that are entirely open and represent the cuesta cliffs. Most of the studied cores reveal wide zones of weak isotopic alteration (tens of cm to several m; 1 to 3 ‰) and narrow zones of strong alteration (4 to 15 mm; up to 4-7 ‰) for both C and O. Interaction between carbonate bedrock and fluids is expected in relatively closed hydrogeological conditions characteristic of the hypogene speleogenesis. The most-altered rock has isotopic composition identical to that of the phreatic speleothems, found in some of the studied conduits. This suggests that the altering waters and mineral forming solutions are genetically related. Several alteration trends are apparent in the C-O isotope space, suggesting that the bedrock has experienced more than one stage of alteration, characterized by different physico-chemical parameters of fluids (including different sources and isotopic composition of carbon and, possibly, different temperatures).