Purpose <p>The commercial <i>In situ</i> forming implants (ISFIs) use either a dual-syringe system requiring pre-injection syringe mixing, or a pre-mixed ready-to-inject syringe. The purpose of this study is to comprehensively evaluate the impact of these two mixing approaches on ISFIs' properties as well as <i>in vitro</i> performance.</p> Methods <p>Computed tomography (CT) imaging and scanning electron microscopy were used to access implant structure ad morphology. High-performance liquid chromatography was used for drug release determination. Gel permeation chromatography was used to evaluate polymer degradation. The CT contrast agent iohexol was used as a model compound and was fully dissolved in the matrix regardless of the mixing approach.</p> Results <p>Syringe-mixed ISFIs contained air bubbles, while ready-to-inject formulation displayed a more uniform polymer matrix without them. The introduction of air bubbles exhibited slower solvent NMP release, altered drug deposition, increased volume expansion, and enhanced initial degradation rate. CT imaging revealed that ready-to-inject formulation was more uniform, whereas syringe-mixed implants appeared to have more heterogenous cavity feature.</p> Conclusions <p>The introduction of air bubbles through pre-injection mixing could potentially impact drug release and ISFI’s performance. This study elucidates the comparison between two mixing techniques as a reference in the developmental process of future products.</p> Graphical Abstract <p></p>