Gel particles obtained from polyethylenimine (PEI) cross-linked with 10% ethylene glycol diglycidyl ether show considerable selectivity in size changes if exposed to solutions of different effector compounds. Monocarboxylic acids induce contractions of, e.g., -9% for benzoate and -40% for 2-naphthoate (all values in one dimension such as length). With diacids, the contractions increase to -29% for 1,4-benzene and -68% for 2,6-naphthalene disulfonate, indicating the significant contributions of noncovalent ionic cross-linking and of interactions with aromatic residues. A striking cooperativity is observed if aromatic compounds such as naphthoic acid are used simultaneously with amino acids as effectors. The combination with, e.g., 2-naphthoic acid and phenylalanine induces, e.g., 69% contraction, whereas the single effector compounds induce only 40% and 8%, respectively. As expected with contraction of chemomechanical polymers, the kinetics of effector absorption is significantly slower than that of the size changes, which take, e.g., 5 min to reach one-half of the final contraction; with smaller gel particles, the rates are significantly increased. Gravimetric determinations show that release of solvation water is largely responsible for the observed volume contractions. Copper or zinc ions induce small contractions, also in experiments with PEI solutions, but show no evidence of cooperativity in combination with amino acids or peptides. MAS NMR spectra of the gels induced by aromatic effector compounds exhibited moderate upfield shifts of the polymer backbone signals, as a result of ring current effects.