It has been previously established that breast cancer cells exhibit high expression of the monocarboxylate (lactate) transporters (MCT1 and/or MCT4) and carbonic anhydrase IX (CAIX) and form a functional metabolon for proton-coupled lactate export, thereby stabilizing intracellular pH. CD147 is the MCT accessory protein that facilitates the creation of the MCT/CAIX complex. This study describes how the small molecule Beta-Galactose 2C (BGal2C) blocks the physical and functional interaction between CAIX and either MCT1 or MCT4 in Xenopus oocytes, which reduces the rate of proton and lactate flux with an IC of ~90 nM. This value is similar to the K for inhibition of CAIX activity. Furthermore, it is shown that BGal2C blocks hypoxia-induced lactate transport in MDA-MB-231 and MCF-7 breast cancer cells, both of which express CAIX. As in oocytes, BGal2C interferes with the physical interaction between CAIX and MCTs in both cell types. Finally, X-ray crystallographic studies highlight unique interactions between BGal2C and a CAIX-mimic that are not observed within the CAII active site and which may underlie the strong specificity of BGal2C for CAIX. These studies demonstrate the utility of a novel sulfonamide in interfering with elevated proton and lactate flux, a hallmark of many solid tumors.