Elegantly regal in a grey halter neck gown, 27-year-old Kate hosted a table of guests at an auction for one of her beloved charities - the Starlight Children's Foundation.
As well as her handsome escort William, the Reading-born brunette was supported by parents Michael and Carole and her younger sister, 25-year-old Pippa, at the £100-a-head black tie do held at London's Saatchi Gallery.
To help raise money for the worthwhile cause, Mr and Mrs Middleton donated a party for 20 youngsters as one of the evening's prizes.
It is not the first time that Kate has shown her support for the children's charity.
Last year she helped organise the Boodles Boxing Ball – a society fight night attended by William, his younger brother Prince Harry and his then girlfriend Chelsy Davy.
That evening, which attracted around 800 guests, raised more than £100,000.