The Palm Pre Plus has a slider keyboard, a larger touchscreen and a high-performance processor compared to its predecessor, the Palm Pre.
Meanwhile, at 55x111x11mm and weighing 100g, the Palm Pixi is smaller and thinner than the Palm Pre. It features a 2.6in 320x400 multitouch screen that offers the same gesture control functionality as the Palm Pre.
The Palm Pixi Plus will be available on O2 from May 28
Both handsets run the company's WebOS platform and come with Wi-Fi connectivity, integrated GPS and the ability to access apps from the Palm App Catalogue.
"We are excited to bring Pre Plus and Pixi Plus to the market for O2 customers," said Palm's Jon Rubinstein.
"With the choice of these two new Palm webOS phones, customers across the UK can stay connected, so they never miss a thing."
Both handsets will be available on tariffs priced from £25 per month. The price plans feature unlimited data, unlimited public Wi-Fi at The Cloud and BT Openzone and unlimited texts, along with minute allowances.