Purpose: The rise of cryptocurrencies sparks significant interest among investors, but the environmental impact of energy-intensive consensus mechanisms, particularly in non-green cryptocurrencies like Bitcoin, raises concerns. As a result, a new class of eco-friendly "green" cryptocurrencies is born, to prioritize sustainability issues. The two-fold objectives of the study are to investigate the long-term equilibrium relationships, or cointegration, between prominent non-green cryptocurrencies (Bitcoin, Ethereum, and Litecoin) and green cryptocurrencies (IOTA, Ripple, and Chia), and to identify the potential for environment-friendly pairs trading strategies and the construction of sustainable cryptocurrency portfolios.

Design/Methodology/Approach: The study identifies the top three cryptocurrencies with the highest market capitalization and liquidity: Bitcoin (BTC), Ethereum (ETH), and Litecoin (LTC). It also identifies three of the most environment-friendly (in terms of power consumption) cryptocurrencies: IOTA (IOT), Ripple (XRP), and Chia (CHI). Daily closing prices of the six selected cryptocurrencies are considered from 10 November 2020 to 11 February 2025, providing 1555 data points for each series. The study employs a battery of seven popular cointegration tests to analyse the relationships between the selected green and non-green cryptocurrencies.

Findings: Statistically significant cointegration is observed between certain pairs of green and non-green cryptocurrencies, as well as among some green cryptocurrencies. Chia demonstrates strong long-run relationships with Litecoin and IOTA. Weak cointegration is observed between other pairs, with strength and consistency varying across different tests. The results suggest potential opportunities for market-neutral trading strategies and diversified portfolios that balance environmental and financial goals.

Originality of the Study: This research explores the integration of green cryptocurrencies into investment strategies, offering a new perspective on balancing environmental considerations with financial objectives in the cryptocurrency market. The study also suggests potential avenues for developing sustainable cryptocurrency portfolios and market-neutral trading strategies.