<p>The generalized Maxwell and Kelvin models represent the most general frameworks among classical viscoelastic models, remaining highly prevalent and extensively employed in contemporary studies. However, a primary limitation is that their individual parameters often lack distinct physical significance and suffer from non-uniqueness. This study establishes the physical interpretation of the parameters in the generalized Maxwell and Kelvin solid models from the perspective of intrinsic mechanical properties, applying a recently established theoretical framework based on frequency-domain analysis. The findings reveal that for each generalized solid model, specific algebraic combinations allow the ambiguous individual parameters to be mapped onto three intrinsic mechanical properties of a viscoelastic solid: the modulus of elasticity of the solid part, the bulk modulus of elasticity of the liquid part and the viscosity of the liquid part. Crucially, for a given dataset, while the individual parameters of both generalized solid models are non-unique, are model-dependent and generally lack physical significance, their specific algebraic combinations are mathematically unique, are invariant and are physically significant. These findings are validated by published experimental data on polymer composites, demonstrating that both generalized solid models yield identical physical invariants despite having distinct model parameters. This study transitions the generalized Maxwell and Kelvin solid models from inherently phenomenological descriptions to frameworks enriched with physical insight. Given the foundational significance and enduring prevalence of these generalized viscoelastic models, establishing the physical interpretation of their parameters marks a breakthrough milestone in the theory of viscoelasticity and related disciplines.</p>