The magnetic and transport properties of the compounds Nd 0·5Sr0·5Mn1-xCoxO 3 (x = 0·1, 0·3 and 0·5), synthesized by citrate-gel route have been investigated. The spin transition in cobaltates at low temperatures affects the magnetic as well as transport properties. The irreversibility behaviour between the zero-field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature becomes stronger with increasing Co content. This is understood on the basis of glassy behaviour, which becomes more robust with increasing Co substitution. The non-saturating M-H behaviour indicates strong magnetic inhomogeneities which may cause the magnetic phase separation at the nanoscopic length scale. The double exchange interaction is stronger between Mn3+-O2-Mn4+ as compared to Co3+-O2-Co4+ pairs. Co-substitution suppresses the double exchange which will lead to cluster/spin glass like behaviour as well as semiconducting features due to localization of charge carriers (mobile eg electrons). © Indian Academy of Sciences.