Markup:

            <r:XamRibbon IsMinimized="{Binding IsRibbonMinimized, Mode=TwoWay}" />

Code-behind:

        private bool isRibbonMinimized;
        private bool IsRibbonMinimized
        {
            get { return isRibbonMinimized; }
            set
            {
                if (isRibbonMinimized != value)
                {
                    isRibbonMinimized = value;
                    SendPropertyChanged("IsRibbonMinimized");
                }
            }
        }

Thanks.

Silly me. I did specify the DataContext (and even the binding mode!), but forgot to change the property to public... too much of a copy-paste I guess :)

It works like a charm. Thank you for your patience.

I don't think you can bind to a private property. So as long as your IsRibbonMinimized is public and the object that has that property is the DataContext (since you're not specifying a source/relativesource) then it should work.