The factors that determine the conformation and stability of G-quadruplex forming sequences remain poorly understood. Here we demonstrate the influence of cosolvents on the conformation and stability of the human telomeric sequence d(A(GGGTTA)(3)GGG)) in both K+ and Na+ containing solutions using a combination of circular dichroism, NMR, and thermodynamics. Molecular crowding arguments have previously been used to suggest that the parallel quadruplex form may be biologically relevant. However, the small cosolvents previously used, PEG 200 and 400, are actually dehydrating agents. We have used acetonitrile as a non-hydrogen-bonding dehydrating agent; similar conformational transitions were observed in K+ solution. Moreover, NMR analysis shows that the resulting structure contains non-anti guanine glycosyl torsion angles suggesting that The conformation present in acetonitrile is not identical to the all-parallel crystal structure, despite the supposed parallel type CD spectrum.