The Catholic Diocese of Pittsburgh has reached a sales agreement with a New York real estate firm interested in purchasing St. Nicholas Church on the North Side, a spokesman for the diocese said yesterday.

Father Ronald P. Lengwin said Follieri Group LLC was in the midst of a 90-day "due diligence" period, reviewing the historic property, before the sale would be made final. The sales price is not yet public information, and Father Lengwin said he was unaware of the company's plans for the church.

"We just know they're committed to using the building in an appropriate way, in recognition it always served as a worship site," Father Lengwin said.

Unrelated to the sales agreement, Pittsburgh Historic Review Commission Chairman Michael Eversmeyer sent a letter to the diocese last week reminding it that no alterations to the exterior of the church were permitted without approval of the commission. Concerns had been raised from some citizens about the possibility of the church's stained glass windows being removed, or other features being altered, Mr. Eversmeyer said in an interview yesterday.

"It's not really a substantive issue at this point. It's just a matter of reiterating what the procedure would be," he said. The diocese always has abided by historic status requirements affecting its properties in the past, Mr. Eversmeyer said, and any successors to the property would be bound by the same city historic designation the 107-year-old church received in 2001.

"The commission has no concern over who owns it and how the building is used," he said. "It's just simply and strictly how it looks."

A representative of the Follieri Group did not return a phone call seeking comment. Father Lengwin said the diocese and Follieri were aware of the restrictions on altering the church exterior and will abide by them.

St. Nicholas was the first Croatian Roman Catholic Church in the United States. It closed two years ago for financial reasons, with its congregation moving to another Croatian parish in Millvale. That parish will receive any proceeds from sale of the church, Father Lengwin said.

The Croatian American Cultural and Economic Alliance sought to buy the church last year to turn it into a cultural center and shrine, but could not reach an agreement with the diocese.

The Follieri Group, which also has been negotiating with the diocese for purchase of other vacant buildings, focuses its development efforts on Catholic church properties across the nation that have outlived their religious needs. Its Web site says it typically seeks to reuse buildings for socially responsible purposes such as affordable housing or community centers.