The organelle that produces H2O2 (hydrogen peroxide) as a by-product is the peroxisome.

Here's why:

* Peroxisomes are small, membrane-bound organelles that contain enzymes involved in various metabolic reactions. One of their key functions is the breakdown of fatty acids through a process called beta-oxidation. This process generates hydrogen peroxide (H2O2) as a byproduct.

* Other organelles like mitochondria are involved in cellular respiration, which does not produce H2O2 as a main product.

Let me know if you'd like more information about peroxisomes or their functions!