Olefination of dialkyl squarates 1–5 with several ylides 6 and phosphonates 7 gave the corresponding alkylidene products 8–12. In the case of Horner-Emmonsreactions, stereoselective formation of Z-alkylidene products was observed. An acid-catalyzed hydrolysis of 8–12 gave squarylacetic acid esters 13.