Acid casein powder was used to prepare caseinate solutions (27 g/kg) with different ionic milieus: sodium caseinate (NaCN, I ∼0.015 mol/L) and calcium caseinate (CaCN, I ∼0.03 mol/L) were obtained by neutralisation with NaOH and Ca(OH)₂, respectively, and dissolving in phosphate buffer resulted in a high ionic strength caseinate solution (CN-PB, I ∼0.16 mol/L). Treatment with microbial transglutaminase (mTGase) for defined incubation times lead to different extents of casein cross-linking, which were characterised by size exclusion chromatography and the N-ε-(γ-glutamyl)-lysine isopeptide content (IC). Maximum polymer size was reached at ∼90% casein polymerisation, and increased in the order NaCN < CN-PB < CaCN. Further enzyme treatment, however, increased the IC, pointing to cross-links within existing polymers. We suggest that the maximum polymer size is determined by the size of casein particles resulting from self-assembly in solution and that mTGase preferably acts on molecules within the same particle. Enhanced association at higher ionic strength (CN-PB) or in the presence of bivalent cations (CaCN) may therefore result in larger covalently cross-linked casein polymers. Furthermore, oscillation rheometry revealed that the relationship between casein cross-linking and stiffness of gels acidified with glucono-δ-lactone depends on the ionic milieu. While for NaCN G’_MAX increased with the time of cross-linking, the presence of ions resulted in the highest G’_MAX at moderate cross-linking intensities. This was also observed when NaCl was added to cross-linked NaCN. The results suggest that electrostatic attraction during gel formation are interfered by ions and cannot be compensated by rearrangements in case of extensively cross-linked casein particles.