The magnetic properties of Bi1- xCaxFe1- yTiyO3-δ (BCFO: y = 0 and BCFTO: x = y) nanoparticles are studied across a wide range of temperatures (20 K to 960 K) for different Ca (and Ti) concentrations [x (= y) = 0, 0.025, 0.05, and 0.1]. X-ray diffraction and electron microscopy revealed the gradual emergence of the orthorhombic phase (Pnma) with an increase in the Ca2+ content in BCFO, contrary to the retention of parent rhombohedral symmetry (R3c) in Ca2+-Ti4+ co-doped BCFTO. XPS indicates the presence of 3+ valence states for Bi and Fe and under-coordinated defect peaks in O 1s spectra. The ordering of oxygen vacancies in BCFO affects the FeO6 octahedral alignment, resulting in a systematic shift of Fe-O Raman modes. Oxygen vacancies formed due to Ca2+ doping in BCFO and the non-magnetic Ti4+ ion at the Fe3+ site in BCFTO disrupt the spin-cycloid propagation in BiFeO3, largely influencing the magnetic properties. These substitutional changes, in addition to the large surface area, are the sources of net magnetization in these systems. Magnetic hysteresis and field dependent zero field cooled-field cooled curves indicate the combined presence of anti-ferromagnetic and ferromagnetic components in BCFO and BCFTO nanoparticles. High temperature magnetic studies present a clear bifurcation of magnetic Néel transition centered at ∼600 K associated with the structural variation in BCFO. A strong anomaly observed at 860 ± 40 K in all the samples suggests a Hopkinson-like effect arising due to sudden loss of anisotropy by the FM component. © 2018 Author(s).