 <p>Just add <code>weight_1</code> and <code>weight_2</code>. igraph does not currently have a way to combine vertex/edge attributes from multiple graphs, except by hand. This is usually not a big issue, because it is just an extra line of code (per attribute). Well, three lines if you want to remove the <code>_1</code>, <code>_2</code> attributes. So all you need to do is:</p> <pre><code>E(g3)$weight &lt;- E(g3)$weight_1 + E(g3)$weight_2 </code></pre> <p>and potentially</p> <pre><code>g3 &lt;- remove.edge.attribute(g3, "weight_1") g3 &lt;- remove.edge.attribute(g3, "weight_2") </code></pre> <p>I created an issue for this in the igraph issue tracker, but don't expect to work on it any time soon: <a href="https://github.com/igraph/igraph/issues/800" rel="nofollow">https://github.com/igraph/igraph/issues/800</a></p>