Shares of American Express (NYSE: AXP) plummeted more than 6% in last Friday's morning trading. What's going on? Well, the iconic charge card company issued its second-quarter results, and while revenue and earnings growth were strong, rising expenses worried investors.

Amex reported revenue net of interest expense of $19.6 billion, 10% higher than the same period a year ago. That was driven by higher card member marketing expenses -- up about 9% -- during the quarter. Earnings per share rose 11%, to $4.53, about $0.12 higher than analysts expected.

But there was one thing the market really didn't like. The company said expenses grew 12% year over year in the quarter to $14.5 billion, from $12.9 billion a year ago. That higher level will continue through the end of 2026, CFO Christophe Le Caillec said on a call with analysts.

He expects marketing expenses to be 10% higher in the second half of the year. The company has been increasing its marketing spending on several card products to attract and retain members.

Amex has had success with younger consumers, including millennials and Gen Z, who are its fastest-growing group. That's a real positive. Yet, the higher marketing expenses -- both in the second quarter and for the remainder of the year -- could indicate that those new memberships are increasingly expensive for the company to obtain.

American Express is an advertising partner of Motley Fool Money. Matthew Benjamin has no position in any of the stocks mentioned. The Motley Fool has positions in and recommends American Express. The Motley Fool has a disclosure policy.

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