The reaction between a 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin-FeCl linker (TCPP-Fe) and lanthanide ions (Ln³⁺) in excess of l-proline coligand provides a synthetic route to two structurally different metal-organic frameworks (TCPP-FeOH_0.5/H2O_0.5)(l-proline)₂Ln₂(H2O)(DMF)_0.5 (where Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺) and (TCPP-FeOH)(l-proline)Ln_1.5 (where Ln = Gd³⁺, Tb³⁺ Tm³⁺ and Yb³⁺). The selection between the two different structures is dependent on the lanthanide ion atomic number. From the different Ln³⁺ ions used in this study, early-to-mid lanthanides, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺, give a structure consisting of discrete Ln₈ building units (1-Ln), while mid-to-late lanthanides, Gd³⁺, Tb³⁺, Tm³⁺, and Yb³⁺, give a framework built upon one-dimensional Ln³⁺ chains (2-Ln). Therefore, the size of the lanthanide ion seems to play a key role in the structure selection and stability, which contrast with the commonly accepted behavior of lanthanides. Activation and subsequent argon gas sorption analyses done using 1-Nd and 2-Tb showed that 1-Nd is permanently porous with a determined surface area of 1223 +/- 4 m²/g, while 2-Tb undergoes a structural change significantly decreasing its surface area (236 m2/g) from its expected value (ca. 900 m²/g). Stability tests on the activated samples revealed that 1-Nd lost its crystallinity after 1 month of exposure to atmospheric moisture, whereas 2-Tb retained its crystallinity, underscoring the higher long-term stability of 2-Tb compared to that of 1-Nd.