The alkali cation (Li+, Na+, K+, Rb+, and Cs+) binding properties of cryptands [2.1.1], [2.2.1], and [2.2.2] were investigated under solvent-free, gas-phase conditions using Fourier transform ion cyclotron resonance mass spectrometry. The alkali cations serve as size probes for the cryptand cavities. All three cryptands readily form 1:1 alkali cation complexes. Ligand−metal (2:1) complexes of [2.1.1] with K+, Rb+, and Cs+, and of [2.2.1] with Rb+ and Cs+ were observed, but no 2:1 complexes of [2.2.2] were seen, consistent with formation of “inclusive” rather than “exclusive” complexes when the binding cavity of the ligand is large enough to accommodate the metal cation. Kinetics for 2:1 ligand−metal complexation, as well as molecular mechanics calculations and cation transfer equilibrium constant measurements, lead to estimates of the radii of the cation binding cavities of the cryptands under gas-phase conditions:  [2.1.1], 1.25 Å; [2.2.1], 1.50 Å; [2.2.2], 1.65 Å. Cation transfer equilibrium studies comparing cryptands with crown ethers having the same number of donor atoms reveal that the cryptands have higher affinities than crowns for cations small enough to enter the cavity of the cryptand, while the crowns have the higher affinity for cations too large to enter the cryptand cavity. The results are interpreted in terms of the macrobicyclic cryptate effect:  for cations small enough to fit inside the cryptand, the three-dimensional preorganization of the ligand leads to stronger binding than is possible for a floppier, pseudo-two-dimensional crown ether. The loss of binding by one ether oxygen which occurs as metal size increases for a given cryptand is worth approximately 25 kJ mol-1, and accounts for the higher cation affinities of the crowns for the larger metals. The Li+ affinity of 1,10-diaza-18-crown-6 is ∼1 kJ mol-1 higher than that of 18-crown-6, while the latter has lower affinity than the former for all of the larger alkali cations (about 7 kJ mol-1 lower for Na+, and about 15 kJ mol-1 lower for K+, Rb+, and Cs+). The equilibrium measurements also allow the determination of relative free energies of cation binding for a number of crown ethers and cryptands. Molecular mechanics modeling with the AMBER force field is generally consistent with the experiments.