We synthesized and characterized two magnetically isolated spin ladders, Cu₂(CO₃)(ClO₄)₂(NH₃)₆ (1) and Cu₂(CO₃)(ClO₄)₂(H₂O)(NH₃)₅ (2), which are the first examples of carbonate bridging molecular spin ladders. Compounds 1 and 2 form a ladder configuration by stacking a structural unit composed of two Cu²⁺ ions and one CO₃²⁻, where the Cu–O–Cu interactions form the rungs and legs of each ladder and the counter anions (ClO₄⁻) occupy the space between the ladders and ensure their magnetic isolation. A S = 1/2 magnetically isolated spin-ladder model with a ladder-rung magnetic interaction J₁/k_B = 364 K (where J is defined as positive for antiferromagnetic interactions) and a ladder-leg magnetic interaction J₂/k_B = 27.4 K accurately predicts the temperature dependence of the molar magnetic susceptibility for 1. The ladder configuration of 2 is similar to that of 1 except that the CO₃²⁻ is alternately skewed in different directions in the stacked structural unit. Interestingly, this minor structural variation in 2 results in its remarkably different magnetic behavior; the magnetic susceptibility curve of 2 is accurately described by an alternating chain model with J₃/k_B = 7.26 K and J₄/k_B = 4.42 K.