Although there were approximately 30 churches in the province of Catalca, situated in the west of Istanbul, prior to the population exchange between Turkey and Greece (1923-1930), only eight of these churches survived. Six of them were preserved by being repurposed as a mosque after the exchange, and two fell into ruin as a result of neglect and disuse. This study is an examination of the conversion process and architectural characteristics of churches in Catalca that were converted into mosques. Having been built based on a basilica plan between the last quarter of the 19th century and the first quarter of the 20th century, most were constructed with brick and stone rubble, and include a three-nave hipped roof. The establishment of a museum that will support further research of the unknown history of these structures would help to introduce the conversion process and the architectural features of these cultural assets to the public.