Polycationic ammonium or phosphonium salts (I) are new. Salts (I) comprising a polycation of formula (Ia) and the anion of a weak or strong (in)organic acid are new: X = N or P; k = 1-24; B = an optionally substituted divalent group selected from (CH2)n(OCH2)m and (CH2)n(OCH2)m-Ar-(OCH2)l(CH2)p; Ar = phenylene, naphthylene or Ar'-(CH2)q-Ar'; Ar' = p-phenylene; n, m, l, p, q = 0-6, but n and m are not both 0 in (CH2)n(OCH2)m; R1-R3 = H or optionally substituted groups selected from 1-6C alkyl, 2-6C alkenyl, 2-6C alkynyl, phenyl, benzyl, (CH2)mZ(CH2O)nPh and (CH2)m(CH2O)nPh; Z = O, CO, COO, OCO, CONH, NHCO, OCONH, NHCOO, S or S-S; Ra, Rb, Rc = H or optionally substituted groups selected from 1-21C alkyl, 2-21C alkenyl, 2-21C alkynyl and (CH2)xZ(CH2)yCH3, or Ra+Rc is optionally substituted 1-8C alkylene, 2-8C alkenylene or 2-8C alkynylene; x, y = 0-24. Independent claims are also included for the following: (1) a method for stabilizing and/or isolating nucleic acids from a biological sample, comprising contacting the sample with (I); (2) complexes (II) comprising nucleic acids and (I); (3) pharmaceutical, diagnostic and research compositions comprising (I) or (II).