The interactions of two ionic surfactants on the decay of ornamental stone (porous limestone) by salt crystallization, a common and damaging weathering process, were studied. Conductivity and/or surface tension measurements allowed calculations of the critical micellar concentration (cmc) of sodium dodecyl sulfate (SDS) and cetyldimethylbenzylammonium chloride (CDBAC) in distilled water and saturated sodium sulfate solution (both with and without the addition of calcite), total surfactant adsorption onto calcite (Gamma(tot)), and the area (A(s)) which a surfactant molecule occupies at the liquid-air interface. In aqueous calcite suspensions SDS shows a strong cme reduction due to Ca2+ binding to the micelles that undergo sphere-to-rod transition at SDS concentrations > cmc, while calcium dodecyl sulfate (Ca(DS)(2)) precipitation contributes to a reduction of DS-concentration in the bulk solution. Adsorption of DS- on calcite is promoted in the saturated saline solution where Gamma(tot) reaches values of 1 x 10(-2) mmol m(-2). CDBA(+) is preferentially adsorbed onto calcite in water (Gamma(tot) = 1.89 x 10(-3) mmol m(-2)) while adsorption on calcite in saturated sodium sulfate solution is limited (Gamma(tot) = 2.18 x 10(-4) mmol m(-2)) due to competition with Na+ for calcite adsorption sites. Limited CDBA(+) adsorption onto calcite, and significantA(s) reduction in the saline solution, results in sphere-to-rod (or disk) micelle shape transition. The previous results together with in situ, high magnification environmental scanning electron microscopy (ESEM) studies and macroscale salt crystallization tests revealed that: (a) the adsorption behavior of DS- induces crystallization of mirabilite at high supersaturation, resulting in nonequilibrium crystal shapes that promote significant damage to the stone; (b) rodlike and/or disc-shaped CDBA(+) micelles enhance solute solubilization and transport to mirabilite nuclei growth sites, inducing crystallization of euhedral crystals formed at low supersaturation and distributed homogeneously throughout the stone pore system. While CDBAC initially reduces stone damage by salt crystallization, it can ultimately result in enhanced damage when in contact with water due to mechanical weakening of the stone and rehydration of previously dehydrated mirabilite crystals within the stone pores. The implications of these results in the conservation of ornamental stone are discussed.