Law firm Reed Smith LLP sued the Cook County Revenue Department and director Zahra Ali over the personal property use tax, alleging that the statute effective last month is unconstitutional.

The 1.25 percent tax applies to personal property other than motor vehicles that was purchased outside the county by Cook County residents, not subject to Cook County sales tax and first subjected to use in the county.

In any year, a taxpayer could exempt as much as $3,500 (for a credit of as much as $43.75) on the first purchase that is subject to the tax.

In addition to raising money for Cook County government, the tax, by increasing the relative cost of items purchased outside the county, is designed to stimulate economic activity within the county.

But Reed Smith, a Pittsburgh-based firm whose Chicago office grew out of Sachnoff & Weaver, says that value-based property taxes are not allowed under the Illinois Constitution unless authorized by the General Assembly, as they are for Cook County on vehicle purchases outside the county.

A county spokesman said of the ordinance,"We're confident this falls within our home rule authority" and promised the defendants would "vigorously defend" themselves against the allegations.

“Like any other taxpayer, we find reasons not to pay it. It's an illegal tax,” said Reed Smith partner Michael Wynne. “We're no different from our clients who received a registration packet from the county.”

In its complaint filed Wednesday in Cook County Circuit Court, Reed Smith said the ordinance does not provide a way to receive a credit against the tax for local sales taxes paid outside Cook County.

Mr. Wynne said clients and others have talked about suing the county, but, to his knowledge, Reed Smith's action is the first.

He noted that the Illinois General Assembly in 1990 outlawed sales or use taxes by home rule units like Cook County based on the purchase price of tangible personal property other than motor vehicles.

"Cook County may argue that its use tax falls outside the Legislature's prohibition, because the tax is nominally based on value,” Mr. Wynne wrote on Reed Smith's website. "However, it seems highly unlikely that a court would respect this distinction, given the fact that, in most cases, the best measure of the value of property when first subject to use within the county would be its purchase price."