The Scottish SPCA is appealing for information and urging cat owners to be vigilant following the alleged poisonings in Newtongrange, Midlothian.
Reports have been received of four possible cat poisonings in the past month, concentrated in and around the town's Fifth Street.
A five-year-old black cat called Lucky died on Monday after showing symptoms of poisoning.
Her owner, who lives on nearby Main Street but did not want to be named, said: "The vets at Dalkeith Veterinary Surgery said that they thought it might have been anti-freeze poisoning. She was having seizures every 15 minutes, and her liver and kidneys failed, which led to her dying at 4pm."
A post-mortem examination is being carried out.