The Diana Cave in SW Romania develops along a fault line at the contact between Late Jurassic limestones and Early Cretaceous marls. It formed through corrosion of bedrock (limestone and marls) by sulphuric acid-rich steam condensate resulted after oxidation/hydrolysis of H2S escaping from the thermo-mineral water emerging from depth in the cave. The sulfuric acid causes a strong acid sulfate weathering of the bedrock generating a sulfate-dominated secondary cave-mineral assemblage that includes gypsum, anhydrite, bassanite, epsomite, alunite, and halotrichite group minerals. Closely associated with these minerals are two rare sulfate species, namely rapidcreekite and tamarugite that represent new occurrences in limestone caves. Traditional X-ray diffraction and single crystal analyses were used along with scanning electron microscope (SEM), stable isotope, and electron microprobe investigations to fully characterize the primary and secondary speleogenetic by-products of Diana Cave.