Nanocrystalline powders of cobalt and aluminum co-substituted zinc ferrites with general formula CoxZn1-xAlFeO4 (x = 0.0-1.0) have been synthesized for the first time. Using the citrate-microwave technique and the citric acid as combustion-complexion agent (fuel), materials with spinel mono-phase cubic spinel structure were successfully prepared. The characterization of products was done by XRD, SEM and VSM. The crystallite size estimated by Scherrer formula has been found in the range of 7.7-9.6 nm. The magnetic properties were studied by room temperature (RT) VSM magnetization measurements. The small remanent magnetization (M-r) and coercivity (H-c) values reveal the superparamagnetic nature of nanoparticles (NPs) at RT. The extrapolated saturation magnetization (M-s) is maximum for Co0.8Zn0.2AlFeO4 (17.15 emu/g) and minimum for ZnAlFeO4 particles (4.22 emu/g). This case is attributed to high or low amount of cation distribution change from normal to mixed spinel structure. The average magnetic diameters (D (mag)) were calculated from magnetic fit studies of M-H spectra. D (mag) values are between 8.17 and 8.46 nm and this range is in great accordance with the obtained diameters from XRD measurements. The small M-r/M-s ratios (maximum, 0.219) specify the uniaxial anisotropy according to Stoner-Wohlfarth model for CoxZn1-xAlFeO4 NPs. RT effective anisotropy constants (K (eff) ) were calculated by using M-s and H-c values. K (eff) constants increased with increasing Co content in the spinel NPs.