Russian news reports said both women are Soviet-born and emigrated to the United States in 1989, but had visited Russia repeatedly since then.
They are believed to have arrived in Moscow in mid-February to attend a wedding and had been hoping to return home soon.
Russian authorities are investigating when and how the women were exposed to the poison - a colourless, tasteless substance that can be fatal in doses of as little as one gram.
Moscow police declined to comment, but the Ekho Moskvy radio said police were investigating cafes and restaurants in the area of the hotel where the women had been staying.
There was no indication of whether the women had business or political interests in Russia that could have made them a target for poisoning.
Thallium has the reputation as being favoured as a murder weapon by assassins.
Thallium was at first suspected in the murder last November of former Russian intelligence agent Alexander Litvinenko in London.
Experts later identified the poison used as radioactive polonium.
The poison, usually in a powdery or crystallised state, works by knocking out the body's supply of potassium, essential for healthy cells, and attacking the nervous system, the stomach and kidneys.
It can also come in radioactive form.
Its effects are not immediately noticeable and frequently take weeks to show up, exhibiting symptoms such as hair loss and a burning sensation in extremities.
In the past, thallium has been used in rat poison and it continues to be used industrially to manufacture products including glass lenses, semiconductors, dyes and pigments.