PEGylated liposomes containing free maleimide functional group for maleimide-thiol conjugation. The liposomes are co-formulated with 1 mol% DSPE-mPEG2000-maleimide, and supplied in a pH-optimized buffer for direct conjugation of the thiol-containing molecule of choice. Conjugation can be achieved by incubating the liposomes with the activated thiol-containing compound which has been reduced by a disulfide bond reducing agent such as (tris(2-carboxyethyl)phosphine) (TCEP), which can be purchased as an add-on.