Energy, natural resources and basic materials proved to be a winning combination of asset categories, as funds in those areas dominated the list of best performing exchange traded funds for May.

Seven of the funds on the accompanying table of top performing ETFs for the month cover a wide array of the energy/natural resources/basic materials fields. The

Market Vectors Russia ETF

(RSX) - Get Report

arguably belongs with the group, as its top holdings include the energy giants Lukoil, Rosneft Oil and Gazprom.

With oil and natural gas making up 39.6% of its portfolio and metals amounting to 24.5% of its holdings, energy and basic materials account for just under two-thirds of the RSX's total net assets.

Specialized energy ETFs on the list include coal (the

Market Vectors Coal ETF

(KOL) - Get Report

), gasoline (the

United States Gasoline Fund LP

(UGA) - Get Report

) and nuclear energy (

Market Vectors Nuclear Fund ETF

(NLR) - Get Report

).

As would reasonably be expected with energy, natural resources and basic materials, the international diversification is a natural byproduct of these funds. KOL, for example, is 45% invested internationally, while more than 80% of NLR's holdings are domiciled outside the U.S.

KOL's largest domestic investments are

Consol Energy

(CNX) - Get Report

,

Peabody Energy

(BTU) - Get Report

and

Joy Global

(JOYX)

.

For NLR, the biggest U.S. holdings include

Exelon

(EXC) - Get Report

,

Constellation Energy Group

(CEG)

and

Cameco

(CCJ) - Get Report

.

The trio of technology ETFs that share the top-10 ETF list with the energy/natural resources/basic materials funds also represent diversification in that sector. Telecommunications and semiconductor ETFs share the stage with a holdover from the tech bubble "business-to-business internet" days -- the

HOLDRS B2G Internet

(BHH)

.

The top holdings of the leveraged

ProShares Ultra Telecommunications ETF

(LTL) - Get Report

include

AT&T Inc.

(T) - Get Report

,

Verizon Communications

(VZ) - Get Report

and

Sprint Nextel

(S) - Get Report

.

The major investments of the leveraged

ProShares Ultra Semiconductors ETF

(USD) - Get Report

are

Intel Corp.

(INTC) - Get Report

,

Texas Instruments

(TXN) - Get Report

TXN and

Applied Materials

(AMAT) - Get Report

.

Richard Widows is a senior financial analyst for TheStreet.com Ratings. Prior to joining TheStreet.com, Widows was senior product manager for quantitative analytics at Thomson Financial. After receiving an M.B.A. from Santa Clara University in California, his career included development of investment information systems at data firms, including the Lipper division of Reuters. His international experience includes assignments in the U.K. and East Asia.