  Cryptoassets such as cryptocurrencies and tokens are increasingly traded on
decentralized exchanges. The advantage for users is that the funds are not in
custody of a centralized external entity. However, these exchanges are prone to
manipulative behavior. In this paper, we illustrate how wash trading activity
can be identified on two of the first popular limit order book-based
decentralized exchanges on the Ethereum blockchain, IDEX and EtherDelta. We
identify a lower bound of accounts and trading structures that meet the legal
definitions of wash trading, discovering that they are responsible for a wash
trading volume in equivalent of 159 million U.S. Dollars. While self-trades and
two-account structures are predominant, complex forms also occur. We quantify
these activities, finding that on both exchanges, more than 30\% of all traded
tokens have been subject to wash trading activity. On EtherDelta, 10% of the
tokens have almost exclusively been wash traded. All data is made available for
future research. Our findings underpin the need for countermeasures that are
applicable in decentralized systems.
