Mixed dimer and mixed oligomer compositions are disclosed. The mixed dimer compositions comprise a monosulfonated cross-dimer or a salt thereof. The monosulfonated cross-dimer is a reaction product of (a) an alpha-olefin sulfonic acid (AOS acid); and (b) an unsulfonated olefin, an unsulfonated olefin precursor, or a functionalized olefin. The mixed oligomer compositions comprise a mono- or polysulfonated cross-oligomer or a salt thereof. The mono- or polysulfonated cross-oligomer is a reaction product of (a) an AOS acid; and (b) an unsulfonated diolefin or an unsulfonated diolefin precursor. Various methods for making the mixed dimer or oligomer compositions are described. Salts of the mixed dimer and oligomer compositions are useful surfactants for foams used in oilfield and other applications. The foams have improved high-temperature stability when compared with foams from AOS dimer acid salts.