In zero magnetic field the recombination of the triplet radical ion pair (RIP) states (3)[P-.+-Sp-Vi(.+)] of zinc porphyrin-viologen dyads with the connecting flexible [(CH2)(m), n = 6, 10] or semirigid [CH2-(Ph)(m)-CH2, m = 1, 2] spacers (Sp) between Zn porphyrin (P) and viologen (Vi(2+)) is a very sensitive analytical tool for detecting the solid-fluid phase transition of the dipalmitoyl (DPPC) or dimyristoyl (DMPC) phosphatidylcholine small unilamellar vesicles. The temperature, T-m = 33.5-38.5 degrees C, and the van’t Hoff enthalpy, Delta H-vH = 80-110 kcal/mol of the phase transition in DPPC bilayers are compared with those obtained using other techniques. In the fluid membranes (at T > T-m) the enhancement of the recombination rates in zero magnetic fields is more pronounced in the case of the flexible (3)[P-.+-(CH2)(n)-Vi(.+)] than in the case of the semirigid (3)[P-.+-CH2-(Ph)(m)-CH2-Vi(.+)]; the corresponding activation energies are E(a) = 2.7 and 7.1 kcal/mol (n = 6, 10) and 0 and 1.1 kcal/mol (m = 1, 2), respectively. The variation of the chain length between the hydrophilic viologen moiety located in the lipid headgroup-water interface and the hydrophobic porphyrin moiety located within the lipid alkyl chains provides for detecting the changes of "microviscosity" in the transversal membrane plane. "Microviscosity" of the DPPC bilayers decreases with the temperature and the distance from the lipid headgroup-water interface; in the temperature range of 42-70 degrees C "microviscosities" eta = 80-27 cP (n = 6) and 43-5 cP (n = 10) are compared with those obtained using other techniques. Application of an external magnetic field (B less than or equal to 0.21 T) results in the pronounced retardation of the RIP recombination rates by a factor of 2-11. In a high magnetic field (ca. 0.2 T) the abrupt changes of the recombination rates associated with the phase transitions are not observed; the corresponding E(a) = 0-3.5 kcal/mol at B = 0.21 T. The magnetic field effects on the recombination rates are discussed in terms of the interplay of spin and chain dynamics modulated by the phase transition and "microviscosity" of phospholipid bilayers.