I made a solemn promise that I would return.

我郑重承诺我会回来的。

《牛津词典》

She made a solemn promise to him when they became engaged that she would give up cigarettes for good.

他们订婚时，她向他许下郑重的承诺，说她会永远戒烟。

《柯林斯英汉双解大词典》

Today, I want to make a solemn promise.

他向我作出了庄严的保证。

youdao

So much for the solemn promise made by BBC Trust chairman Lord Patten, at the beginning of the internal inquiry, to be 'open and transparent'.

BBC: Savile inquiry: Media reaction to BBC transcripts

They breached the solemn promise of five generations of U.S. Treasury officials and set in motion the worldwide credit bubble of the pax dollarum age.

FORBES: It's Been 40 Years Since Richard Nixon's Most Heinous Crime

Let me make you a solemn promise for 2008.

FORBES: We're Too Gloomy