Alginate-gelatin composite gels are widely used in food and biomedical applications. These composite gels are particularly interesting for their ability to establish complex functionalities, such as in bioinks for 3D bioprinting. In order to understand and optimise their techno-functionalities, it is crucial to study the structure formation, which is influenced by molecular interactions and phase behaviour. Therefore, the aim of this study was to characterise the effect of mixing ratio and temperature (21 and 40 °C) on the phase behaviour and structure formation of alginate-gelatin composite gels using internal alginate gelation with Ca-EDTA and D-glucono-δ-lactone. We compared pure alginate gels and composite gels using oscillatory rheology, pH measurements during gelation and microstructural analysis through confocal laser scanning microscopy. The temperature-dependent state of the gelatin - liquid or gelled - strongly influenced the gelation velocity of alginate. At 40 °C, liquid gelatin delayed the gelation process by buffering the pH decrease. This effect was most pronounced in the presence of excess gelatin, which had a strong antagonistic effect on gel strength. At 21 °C, gelled gelatin resulted in a faster alginate gelation and the formation of phase-separated networks at all mixing ratios. This was also accompanied by antagonistic effects on the gel strength. The phase-separated networks showed different microstructures with high-contrast regions depending on the mixing ratio, which correlated with variations in the turbidity of the gels. These insights into the structure-function relationship of alginate-gelatin composite gels may enable the customisation of specific network types for the use in 3D bioprinting.