With the aim of better understanding the phase behavior of alkyl maltosides (n-alkyl-β-D-maltosides, C_nG₂) under the conditions of membrane protein crystallization, we studied the influence of poly(ethylene glycol) (PEG) 2000, a commonly used precipitating agent, on the critical micelle concentration (CMC) of the alkyl maltosides by systematic variation of the number n of carbon atoms in the alkyl chain (n = 10, 11, and 12) and the concentration of PEG2000 (χ) in a buffer suitable for the crystallization of cyanobacterial photosystem II. CMC measurements were based on established fluorescence techniques using pyrene and 8-anilinonaphthalene-1-sulfonate (ANS). We found an increase of the CMC with increasing PEG concentration according to ln(CMC/CMC₀) = k_Pχ, where CMC₀ is the CMC in the absence of PEG and k_P is a constant that we termed the “polymer constant”. In parallel, we measured the influence of PEG2000 on the surface tension of detergent-free buffer solutions. At PEG concentrations χ > 1% w/v, the surface pressure π_s(χ) = γ(0) − γ(χ) was found to depend linearly on the PEG concentration according to π_s(χ) = κχ + π_s(0), where γ(0) is the surface tension in the absence of PEG. Based on a molecular thermodynamic modeling, CMC shifts and surface pressure due to PEG are related, and it is shown that k_P = κc(n) + η, where c(n) is a detergent-specific constant depending inter alia on the alkyl chain length n and η is a correction for molarity. Thus, knowledge of the surface pressure in the absence of a detergent allows for the prediction of the CMC shift. The PEG effect on the CMC is discussed concerning its molecular origin and its implications for membrane protein solubilization and crystallization.