Yes they are, Lisa. Peace lily (Spathiphyllum sp.) is considered to be toxic to pets—and if any part of the plant is chewed or ingested, it can cause significant mechanical irritation and swelling of the oral tissues and other parts of the GI tract. This is true for both cats and dogs.
To get a bit technical, these plants contain cells known as idioblasts. Idioblasts contain raphides, which are slender, spearlike parts of calcium oxalate crystals. Raphides are sharp and needle-shaped, and are packed in a gelatinous substance. When the tip of the idioblast is broken, sap from the plant—or saliva from an animal—enters the cell, causing the gelatin to swell. The swelling action forces the raphides to shoot from the cell, kind of like a gun discharging a bullet. The calcium oxalate crystals penetrate an animal’s oral mucosa, tongue, and throat, causing damage. The cells may continue to expel crystals for a significant amount of time, even after a piece of plant material is swallowed. As this is happening, proteolytic enzymes stimulate the release of kinins and histamines by the body. The rapid inflammatory response from the release of these substances aggravates the damage caused by the crystals. Chewing, bruising, tearing, or otherwise damaging the plant is necessary to produce these effects.
According to our experience at the APCC, most animals exhibit GI-related symptoms, including drooling, nausea, vomiting, diarrhea, from ingesting peace lilies. On rare occasions, oropharyngeal swelling can be severe. Tongue swelling, trouble swallowing, and difficulty in breathing may be seen in these cases. Based on this information, we definitely advise keeping peace lily plants out of the reach of pets.