The complexation process and underlying mechanisms that rule the interaction of DNA with the cationic block copolymer Tetronic T901 to form polyplexes and their potential transfection efficiency have been studied under different solution conditions. We noted that T901 favors the formation of self-assembled structures with partially condensed DNA at smaller polymer concentrations than other Pluronic (TM)/Tetro nic (TM)-type copolymers previously analysed. The observed polyplexes display sizes from the nano- to the micro- range as derived from DLS, electronic and optical microscopies. Also, copolymer micelles are observed at concentrations below the copolymer critical micellar concentration (cmc) induced by the presence of DNA. The complexation process is dependent on solution conditions, with electrostatic and ionic interactions being more important at acidic pH thanks to the predominant diprotonated form of the block copolymer which is less aggregation-prone, whilst dispersive forces are increasingly enhanced under basic conditions or when rising the solution temperature. Whatever the case, the complexation is mainly governed by entropic contributions, as denoted from ITC data. In vitro transfection experiments after complexing T901 with a pDNA encoding the expression of green fluorescein protein, GFP, show a relative successful fluorescence of transfected HeLa cells, which confirms the uptake, internalization and release of the genetic material within the cells at suitable [N]/[P] ratios with relatively low cytotoxicity. Despite the observed successful outcomes, the obtained transfection efficacies are slightly lower than those obtained with Lipofectamine2000, so further optimization of the polyplex formation conditions is envisaged in future studies. (C) 2018 Elsevier Inc. All rights reserved.