Magnetic, optic and microstructural properties of ultrasonically synthesized CoEuxFe2-xO4 (x <= 0.1) nanoferrites (NFs) have been examined in this study. After sonochemical synthesis, XRD and FT-IR analyses confirmed the purity, the structure (cubic spinel structure and Fd3m space group) and the spectral properties of the spinel ferrite samples. The spherical morphology and chemical compositions of the products were observed via transmission and scanning electron microscopes along with EDX and elemental mapping. Percent diffuse reflectance (%DR) was used for optical investigation. Optical band gaps (E-g) were estimated utilizing Kubelka-Munk theory and Tauc equation. E-g values are in a narrow band of 1.34 to 1.44 eV. The magnetic parameters like M-s (saturation magnetization), SQR = M-r/M-s (squareness ratio), n(B) (magnetic moment), H-c (coercivity) and M-r (remanence) have been evaluated by analyzing measurements of magnetization versus magnetic field performed at room (RT; T = 300 K) and low (T = 10 K) temperatures. It is showed that the different produced CoEuxFe2-xO4 (0.00 <= x <= 0.10) nanospinel ferrites present superparamagnetic (SPM) nature at RT. At low temperature, the various produced CoEuxFe2-xO4 (x <= 0.08) nanospinel ferrites display ferrimagnetic (FM) nature. With exception, the x = 0.10 sample exhibit SPM behavior at T = 10 K. It is noticed that the Eu3+ substitutions alter in a significant way on the magnetic data. A decreasing trend in the M-s, M-r and n(B) values was noted with Eu3+ substitutions.