Positron annihilation lifetime spectroscopy has been applied in mixtures and in solutions of solid Al(W), Al(acac),, and Co(III), Co(acac)g, tris(acetylacetonates). For the solid solutions of Co(acac), in Al(a~+c)~, a strong inhibition of positronium formation is observed with increasing cobalt complex concentration, while there is no quenching of the positronium lifetime. In the solid mixtures the positronium ...