Fine, hollow, spherical particles with an average size of one micronmeter were formed through pyrolysis of aerosol of nickel nitrate solution. The aerosol product obtained by fast heating to 450°C consisted of tiny crystallites of NiO, hydrated nitrate, and other non-crystalline phase(s). By heating above 480°C, it consisted only of crystalline NiO. To understand the reactions occurring during the aerosolization process, thermal analysis was conducted on nickel nitrate hexahydrate in stagnant and flowing air using different heating rates. The mechanism for the decomposition of nickel(II) nitrate hexahydrate is proposed.