The reactions of Ln(NO3)(3) (Ln = La, Er) with 1,4-phenylendiacetic acid (H(2)PDA) under hydrothermal conditions produce isostructural lanthanide coordination polymers with the empirical formula [Ln(2)(PDA)(3)(H2O)].2H(2)O. The extended structure of [Ln(2)(PDA)(3)(H2O)].2H(2)O consists of Ln-COO triple helixes cross-linked through the -CH2C6H4CH2- spacers of the PDA anions, showing 1 D open channels along the crystallographic c axis that accommodate the guest and coordinated water molecules. Evacuation of [Er-2(PDA)(3)(H2O)].2H(2)O at room temperature and at 200 degreesC, respectively, generates [Er-2(PDA)(3)(H2O)] and [Er-2(PDA)(3)], both of which give powder X-ray diffraction patterns consistent with that of [Er-2(PDA)(3)(H2O)].2H(2)O. The porosity of [Er-2(PDA)(3)(H2O)] and [Er-2(PDA)(3)] is further demonstrated by their ability to adsorb water vapor to form [Er-2(PDA)(3)(H2O)].2H(2)O quantitatively. Thermogravimetric analyses show that [Er-2(PDA)(3)] remains stable up to 450 degreesC. The effective pore window size in [Er-2(PDA)(3)] is estimated at 3.4 Angstrom. Gas adsorption measurements indicate that [Er-2(PDA)(3)] adsorbs CO2 into its pores and shows nonporous behavior toward Ar or N-2. There is a general correlation between the pore size and the kinetic diameters of the adsorbates (CO2 = 3.3 Angstrom, Ar = 3.40 Angstrom, and N-2 = 3.64 Angstrom). That the adsorption favors CO2 over Ar is unprecedented and may arise from the combined differentiations on size and on host-guest interactions.