Two-dimensional (2D) hybrid organic-inorganic halide perovskites are a preeminent class of low-cost semiconductors whose inherent structural tunability and attractive photophysical properties have led to the successful fabrication of solar cells with high power conversion efficiencies. Despite the observed superior stability of 2D lead iodide perovskites over their 3D parent structures, an understanding of their thermochemical profile is missing. Herein, the calorimetric studies reveal that the Ruddlesden-Popper (RP) series, incorporating the monovalentmonoammonium spacer cations of pentylammonium (PA) and hexylammonium (HA): (PA)(2) (MA)(n-1)PbnI3n+1 (n = 2-6) and (HA)(2) (MA)(n-1)PbnI3n+1 (n = 2-4) have a negative enthalpy of formation, relative to their binary iodides. In contrast, the enthalpy of formation for the Dion-Jacobson (DJ) series, incorporating the divalent and cyclic diammonium cations of 3- and 4- (aminomethyl)piperidinium (3AMP and 4AMP respectively): (3AMP)(MA)(n-1)PbnI3n+1 (n = 2-5) and (4AMP)(MA)(n-1)PbnI3n+1(n = 2-4) have a positive enthalpy of formation. In addition, for the (PA)(2) (MA)(n-1)PbnI3n+1 family of materials, we report the phase- pure synthesis and single crystal structure of the next member of the series (PA)(2)(MA)(5)Pb6I19 (n = 6), and its optical properties, marking this the second n = 6, bulk member published to date. Particularly, (PA)(2)(MA)(5)Pb6I19 (n = 6) has negative enthalpy of formation as well. Additionally, the analysis of the structural parameters and optical properties between the examined RP and DJ series offers guiding principles for the targeted design and synthesis of 2D perovskites for efficient solar cell fabrication. Although the distortions of the Pb-I-Pb equatorial angles are larger in the DJ series, the significantly smaller I center dot center dot center dot I interlayer distances lead to overall smaller band gap values, in comparison with the RP series. Our film stability studies on the RP and DJ perovskites series reveal consistent observations with the thermochemical findings, pointing out to the lower extrinsic stability of the DJ materials in ambient air.