Thallium(I) oxide is a black solid. It reacts with water to make a yellow solution of thallium(I) hydroxide. It reacts with carbon dioxide to make thallium(I) carbonate. It is a strong base. It reacts with acids to make other thallium salts. It is very toxic.
It is made by heating thallium(I) hydroxide or thallium(I) carbonate in air.