Agroindustrial residues represent heterogeneous biomass resources whose anaerobic conversion can be strongly influenced by pretreatment. This study evaluated seven residues: banana peel, potato peel, pineapple peel, coconut fiber, maize cob, coffee parchment, and cocoa parchment, under untreated, organosolv-treated, and precomposted conditions. Physicochemical and bromatological properties were characterized across the three biomass conditions, whereas baseline lignocellulosic composition was determined only for untreated materials. Methane production and biogas composition were assessed for the organosolv-treated and precomposted residues. Organosolv and precomposting produced contrasting compositional responses. Precomposted materials showed lower chemical oxygen demand and volatile-solid proportions, whereas organosolv largely preserved the organic fraction and increased the relative contribution of fiber and carbohydrates. Biochemical methane potential (BMP) differed markedly between pretreatments (p < 0.001), with organosolv-treated residues showing an average BMP 62.2% higher than that of precomposted residues. Methane concentration remained close to 50% and did not differ significantly between pretreatments. The first two principal components explained 79.2% of the compositional variation and showed distinct multivariate patterns among biomass conditions. Exploratory calculations based on experimental BMP and solids data estimated methane recoveries of 305.25 and 155.74 Nm³ CH₄ t⁻¹ wet biomass for organosolv-treated and precomposted materials, respectively. Under the conditions evaluated, organosolv showed greater potential for methane-oriented valorization and provides a basis for subsequent semi-continuous and pilot-scale evaluation.