 <p>It seems like you can't. You can only go as far as ...</p> <pre><code>CompositeId() .KeyProperty(x =&gt; x.Id1, "ID1") .KeyProperty(x =&gt; x.Id2, "ID2"); </code></pre> <p>There is no option for type or length. </p> <p>But in version 1.1 there seems to be a possibility</p> <pre><code>CompositeId() .KeyProperty(x =&gt; x.Id1) .KeyProperty(x =&gt; x.Id2, kp =&gt; kp .ColumnName("ID2") .Type(typeof(string))); </code></pre>