Epoxy resins are extensively utilised across diverse applications owing to their superior chemical resistance, thermal stability, and mechanical properties. Traditionally, these resins are cured using amine or anhydride hardeners; however, increasing attention is being directed towards alternative, more sustainable curing agents, notably carboxylic acids. These acids offer environmental advantages and enable simpler processing without the need for catalysts; however, their curing efficiency is highly dependent on their chemical structure. This study examines the curing of epoxy resin using three distinct carboxylic acids—salicylic acid (a monoacid), citric acid (a triacid), and maleic acid (a diacid featuring conjugated double bonds)—in the absence of catalysts. The objective was to assess their effectiveness in facilitating epoxy ring-opening and network formation. Formulations were prepared by combining each acid with the epoxy resin in stoichiometric proportions, followed by thermal treatment. Curing progression was monitored using Fourier-transform infrared spectroscopy (FTIR), while differential scanning calorimetry (DSC) was employed to evaluate thermal behaviour and degree of cure. Furthermore, gel content analysis quantified the insoluble fraction of the cured samples, serving as a direct indicator of crosslink density. The results revealed that both salicylic and citric acids effectively promoted epoxy ring-opening, leading to the formation of crosslinked networks. Citric acid exhibited superior performance, attributed to its three carboxylic groups, resulting in higher gel content and enhanced crosslink density. In contrast, maleic acid failed to promote curing under the applied conditions, likely due to its conjugated double bonds diminishing the nucleophilicity of its carboxylic groups or causing steric hindrance in reactions with epoxy moieties. These findings highlight the critical influence of acid structure on curing efficiency, positioning multifunctional acids such as citric acid as promising sustainable alternatives for catalyst-free epoxy resin curing.